US 5761294

I'll search for authoritative information on this specific patent, including any 2026 litigation activity.

Let me run a couple more targeted checks for litigation/PTAB activity on this specific patent.

Search scope and confidence note

I searched for the literal identifier 5761294 (and the forms US5761294A / 5,761,294 / 08/683,441) against the patent text, Google Patents, USPTO.report, EveryPatent, and Unified Patents' patent portal, plus general web searches for Federal Circuit 2026 docket activity.

On the "CAFC 2026 docket" request: I found no record of any 2026 Federal Circuit docket, appeal, or PTAB proceeding involving US 5,761,294. I could not query PACER/CAFC's docket system directly, so treat this as "not found," not "confirmed none exists." Independently, the reason such an appeal is unlikely: the patent expired for failure to pay maintenance fees / at end of term on 2016-07-18, and Google Patents lists its legal status as "Expired – Lifetime." (Note: the search-engine surfaced hit about a 2026 Federal Circuit appeal, Spero v. Mercedes-Benz USA, and the 2026 Headwater Research v. Verizon appeal involve entirely different patents — 10,894,503 and the Headwater patents — and have no connection to 5,761,294.)

US 5,761,294 — Bibliographic summary

Abstract (verbatim, from the granted patent)

"A method and system of enabling incoming and outgoing call capability for a digital telephone that is limited to analog transmissions include providing a converter that locally supports the protocol of the digital telephone, but that converts digitized signals of voice information, party-specific call messages and call-handling call messages into outgoing analog signals. In one embodiment, one analog signal carries the voice information and a second analog signal carries the party-specific and call-handling call messages, with the first analog signal being transmitted via a public switching telephone network and the second analog signal being transmitted via a data network. Received analog signals are reconverted to digital signals having the appropriate protocol. In another embodiment, the voice information, the party-specific call messages and the call-handling call messages are transmitted along a single digital link between two converters, typically by means of a data network."

Core technical idea in plain language

The patent addresses the "telecommuter problem": a feature-rich digital phone (e.g., an ISDN or proprietary-protocol set that shows caller ID, forwarded-from info, message-waiting lights) normally needs a digital line to the PBX to work. Digital links were expensive and not universally available. The inventors' solution is a converter at both ends that speaks the digital phone's native protocol locally, but splits the traffic for the analog journey:

• Voice goes over the ordinary circuit-switched PSTN (good quality of service, isochronous).
• Call messages — both "call-handling" messages (CONNECT, CONNECT ACKNOWLEDGE, DISCONNECT, RELEASE) and "party-specific" messages (caller ID, trunk number, forward-reason, message-waiting indicator) — go over a packet data network such as the Internet/ISP link.
• At the far end, the two streams are rejoined and reconverted to the digital protocol for the PBX. Synchronization isn't required because the message channel is not time-critical.

FIG. 1/2 = dual-link (PSTN voice + data-network messages). FIG. 3 = single analog link, everything over the data network (lower QoS for voice, but retains the feature-phone messaging that plain VoIP lacked). FIG. 4 = wireless/cellular analog variant.

Independent claims — plain-language overview

There are three independent claims: 1, 5, and 10 (12 claims total; 2–4 depend from 1; 6–9 from 5; 11–12 from 10).

Claim 1 — Method of establishing communication (two-channel method)

A method of connecting a local digital telephone to a remote site that supports digital communication processing, where the communication occurs without digital lines. Four steps:

1. Convert outgoing digital signals from the digital telephone into two analog signals: a voice analog signal and a separate message analog signal.
2. Send the voice analog signal to the remote site over the public telephone switching network.
3. Send the message analog signal to the remote site over a data network.
4. Receive both voice and message analog signals back from the remote site over those same two networks, respectively, and convert them into digital signals compatible with the digital telephone.

Key limitation: the split-transport architecture — voice on PSTN, call messages on data network — and the explicit "no digital lines" premise. (Note: the granted text reads "outdoing digital signals," which is plainly a typographical error for "outgoing"; I have not auto-corrected it, and it is worth flagging that this typo appears in the printed claim.)

Claim 5 — System for enabling communication (two-channel system)

A system between an interface site supporting digital telecom processing and at least one digital telephone limited to analog transmissions, comprising, for each of the telephone and the interface site:

• Conversion means for converting telephone digital signals into outgoing analog voice and analog message signals, and incoming analog voice/message signals back into digital signals;
• First analog lines to the conversion means, to send/receive the analog voice signals over a public telephone network; and
• Second analog lines to the conversion means, to send/receive the analog message signals over a data network.

Plain language: the system-level mirror of claim 1 — a converter pair joined by two separate analog paths, one for voice (public phone network) and one for signaling/feature messages (data network).

Claim 10 — Method of enabling incoming/outgoing call capability (protocol-support method)

A method of enabling incoming and outgoing call capability for a digital telephone limited to analog transmissions, comprising:

• Locally supporting the digital protocol of the telephone, including supporting local digitized voice information, call-handling call messages, and party-specific call messages;
• Converting those local digitized signals into outgoing analog signals and transmitting them to a remote site that reconverts them to digital — with voice information sent via public telephone network lines, and call-handling and party-specific messages sent via a data network; and
• For incoming analog signals directed to the telephone, converting them into local digitized signals of voice information, call-handling messages, and party-specific messages.

Plain language: same split-transport idea as claim 1, but framed around fully preserving the three-channel digital-phone experience (voice + generic call control + feature-phone/display messages) in both directions, with the local converter emulating the phone's native protocol. (Note the granted claim text contains the typo "local digitized signals form said digital telephone" for "from"; again, not auto-corrected.)

Dependent highlights: claim 3 adds protocol imposition on the received signals; claim 4 makes explicit that both party-specific and call-handling messages travel over the data network; claim 6 adds a modem on the message path; claim 9 adds a visual display responsive to user-specific call messages carried in the analog message signals; claim 11 specifies Internet/ISP as the data network (claim 12 then refers to "public and private data network").

Uncertainties I want to flag explicitly

1. Minor date discrepancies across databases (priority 07-17 vs 07-18; grant 06-01 vs 06-02). The granted patent's own printed heading is "Jun. 2, 1998," which I treat as authoritative for the issue date; the filing/priority date printed on the face of the document is 1996-07-18.
2. Assignee chain is a chain of mergers/assignments (Siemens Rolm → Siemens Business Communication Systems → Siemens Information and Communication Networks → Siemens Communications → Siemens Enterprise Communications → Unify → Enterprise Systems Technologies / Unify GmbH & Co. KG). There is also a 2010 security interest granted to Wells Fargo Trust Corporation and later terminated. "Current assignee" listings vary by database.
3. No 2026 CAFC activity found. I cannot rule out an obscure or unindexed filing, but nothing in the searches ties this patent to any 2026 appellate docket. Given the 2016 expiration, active 2026 appellate litigation over 5,761,294 would be unusual and I found no evidence of it.

I'll research litigation involving US patent 5761294 using the sources you specified.

Let me search more targeted litigation databases.

Let me search more specifically across litigation databases.

Litigation Search Results: US Patent 5,761,294

Bottom line: I found no evidence of any litigation involving US Patent 5,761,294.

What I searched and what I found

I queried for the exact number 5,761,294 / US5761294A across the sources you named and general web results. The results returned were patent-record pages, not litigation records:

• Unified Patents portal (portal.unifiedpatents.com/patents/patent/US-5761294-A) — returned only the patent's bibliographic/portfolio page (assignee, inventors, expiration, innovation indices). It did not return a litigation entry or case list for this patent.
• Google Patents (patents.google.com/patent/US5761294) — returned the full patent document, including its forward citations ("Cited By" / "Referenced By"), which are patent citations, not lawsuits.
• Justia Patents — appearances of "5761294" were only as a prior-art citation inside other patents (e.g., US 9,451,084 and US 7,835,344 list "5761294 | June 2, 1998 | Shaffer et al.").
• General / CAFC / District Court queries returned no docket, case caption, or case number tied to this patent. I did not locate a PACER or CourtListener docket naming US 5,761,294.

Important clarification (citations ≠ litigation)

The lengthy "Cited By (69)" table in the patent record is a list of later patents that cite US 5,761,294 as prior art. This is often mistaken for litigation data but is not. None of those entries represent lawsuits against this patent.

The one "litigation-adjacent" hit is unrelated

You asked me not to return similar-number results, but for transparency: some search results showed other patent numbers in litigation (e.g., patents owned by Velos Media, Uniloc, MCOM IP, etc.). Those involve different patent numbers and are therefore outside the scope. I am excluding them.

Known case data

Caveats on my confidence

• I cannot prove a negative. Absence from the searchable sources above is strong but not conclusive evidence that no suit was ever filed.
• This patent was expired as of ~July 2016 (marked "Expired – Lifetime"; Unified Patents lists expiration 2016-07-17). An expired patent is an unlikely infringement-enforcement vehicle, which is consistent with the lack of litigation.
• Record ownership passed through Siemens → Unify GmbH & Co. KG → Enterprise Systems Technologies S.à r.l. (with Atos SE shown as the ultimate parent company). A thorough check would require paid docket tools (e.g., Docket Alarm, Lex Machina, RPX, or a direct PACER search by patent number), which I could not fully execute here.

If you need a definitive negative clearance, I recommend running a direct PACER "party/patent" search and a Docket Alarm / Lex Machina query on 5,761,294, since those full-text docket databases index case captions and complaint exhibits that open-web search often does not surface.

I'll verify the structured block's "no proceedings" default against live sources before writing, since an absence finding needs corroboration.

Two more confirmatory checks — one for the patent-owner entity's PTAB footprint, one for any post-grant/reexam activity under this patent number.

Proceedings overview

Total AIA trial proceedings on US 5,761,294: 0 — zero active, zero with claims invalidated, zero with claims sustained, zero settled, zero institution denials. The structured "PTAB proceedings on file" block (USPTO Open Data Portal ingest) returns an empty AIA trial list, and my independent searches of PTAB decision text, PTAB E2E-style docketing results, and the Unified Patents patent portal produced no petition, institution decision, FWD, or appeal bearing this number.

Bottom-line defensive posture: there is no PTAB record to lean on in either direction. Nothing has been canceled for you, and nothing has been hardened against you — but for reasons set out below, an assertion of this patent today is close to economically worthless regardless.

### No AIA trial proceedings on file

- **Type**: N/A
- **Filed**: N/A
- **Status**: No proceedings on file (USPTO ODP structured data); corroborated by web search 2026-09-28
- **Judge panel**: N/A
- **Petition grounds**: N/A
- **Institution decision**: N/A
- **Final Written Decision**: N/A — no claim of US 5,761,294 has ever been canceled, confirmed, or construed by the Board
- **Settlement / termination**: N/A
- **Appeal**: N/A — no FWD or institution decision exists to appeal, and no CAFC appeal of a PTAB decision on this patent was found
- **Defensive value**: Neutral. You cannot cite a cancellation, but you also face no petitioner-side estoppel, and every prior-art ground remains available in district court.

False positives to ignore (flagged because they are easy to mistake)

Searches for the string '294 return four unrelated patents that are not US 5,761,294. Do not cite any of these as proceedings on this patent:

Strategic summary

Claim status. All twelve claims of US 5,761,294 — independent claims 1, 5, and 10, plus dependents 2–4, 6–9, and 11–12 — are UNTESTED at the PTAB. None is canceled, none is confirmed, none has been amended in a § 316(d) motion to amend. There is no narrowing through IPR to point to, and therefore no "surviving claim set" to map against an accused product. Any invalidity position you build must be built from scratch in district court or in a fresh (and, for the reasons below, probably pointless) post-grant filing.

Estoppel landscape. Because no IPR or PGR was ever instituted, § 315(e)(2) estoppel does not exist against anyone — not against any petitioner, not against any privy, not against any real party in interest. No defendant, alleged infringer, or customer inherits a ground that was "raised or reasonably could have been raised." Every printed-publication and patent-based § 102/§ 103 ground is unencumbered. (Conversely, nothing about this patent has been roadmapped into the public record by a petitioner either.)

Pattern signals. None of the classic signals are present: no serial petitioner, no defensive aggregator (Unified Patents or RPX) in the chain, no patent-owner appeal activity, no parallel district court track. Ownership trod a long corporate path — Siemens Rolm → Siemens Business Communication Systems → Siemens Information & Communication Networks → Siemens Communications → Siemens Enterprise Communications → Unify GmbH & Co. KG → Enterprise Systems Technologies S.à r.l. (Unify/Atos lineage; Unified Patents lists ultimate parent Atos SE) — but there is no indication this patent was ever monetized, and it never appeared in an assertion campaign. The only record event of note after grant is the normal maintenance-fee schedule (4th/8th/12th year payments) and a 2010-11-10 security interest granted to Wells Fargo Trust Corporation Limited, later released 2015-09-09. None of that is PTAB activity.

The absence of any IPR is itself consistent with, and largely explained by, two hard facts: (i) the patent expired — Google Patents records an anticipated expiration of 2016-07-18, and Unified Patents lists expiration 2016-07-17 (a one-day discrepancy worth noting; I flag the conflict rather than resolving it) — and (ii) no one appears to have ever asserted it. Practitioners rarely spend $300k–$500k on an IPR against a patent with no litigation counterparty and no remaining term.

Recommended next steps

1. Do not describe this patent as "PTAB-tested" or "hardened." Nothing has been tested. The correct characterization in a client memo or a demand-letter response is: no AIA trial proceeding has ever been filed against US 5,761,294.

2. Lead with term, not with the PTAB. The patent expired in July 2016 and was marked "Expired – Lifetime" with the statutory 20-year term having run from the 1996-07-18 filing. Under 35 U.S.C. § 286, recovery is barred for infringement committed more than six years before a complaint is filed; a suit filed today (2026-09-28) would have a lookback window reaching only to 2020-09-28, after which no infringement is possible. Any demand on this patent therefore presents a severe (and probably fatal) damages problem for the patent owner, independent of validity. Note also the laches point is a non-starter post-SCA Hygiene, so the § 286 window is the cleaner argument.

3. If you are nevertheless served or threatened, and a live controversy exists, the practical paths are: (a) a declaratory-judgment action on invalidity/non-infringement in district court, where no IPR-driven estoppel or claim-construction record constrains you (FWDs would have been Phillips-based anyway, 37 C.F.R. § 42.100(b)); or (b) a fresh IPR, subject to the § 315(b) one-year bar running from service of a complaint alleging infringement. Note that an IPR against an expired patent is not categorically unavailable, but the Board's practical value here is low and the expense is not justified by the exposure. If you want a cheaper Office-based invalidation route, note that an ex parte reexamination request under 37 C.F.R. § 1.510 remains an avenue for an expired patent — verify current CRU practice on expired patents before relying on it, as I am not highly confident on that point.

4. Verify the null result directly if this memo will support a court filing. Recommended primary sources:

   • PTAB E2E / PTAB Center search: https://ptacts.uspto.gov/ptabweb/pages/search
   • Unified Patents patent page for US-5761294-A: https://portal.unifiedpatents.com/patents/patent/US-5761294-A
   • Google Patents record: https://patents.google.com/patent/US5761294/en
   • CAFC / district docket search on the patent number: https://www.courtlistener.com/ (query 5761294)

   The structured ODP block plus my searches agree, but I cannot prove a negative from open-web sources; a paid docket tool (Docket Alarm, Lex Machina, RPX) or a PACER query on 5,761,294 is the way to close this out definitively.

5. Cross-reference check: the previously generated litigation section reached the same null result — no plaintiff, no defendant, no case number. The two sections are consistent, and the two together support the conclusion that US 5,761,294 has never been adjudicated anywhere: not at the Board, not in district court. The only discrepancy between sections is the expiration date (2016-07-17 vs. 2016-07-18) and the priority date (1996-07-17 per Unified Patents vs. 1996-07-18 per Google Patents and the application filing date); treat 1996-07-18 as the filing/priority date from the authoritative full text, and treat the expiration as on or about 2016-07-17/18, immaterial to the analysis.

I'll research the assignment record for this patent across the sources you named.

Let me pull the specific assignment records and correspondents of record.

Let me try the USPTO Assignment API and the Patent Assignment Search index directly.

I hit the tool-call ceiling before I could pull every correspondent-of-record line, so I'm flagging clearly below which fields are grounded and which I could not retrieve. No assignments are fabricated.

Inventors

Departure pattern: Not present / no evidence. Both inventors assigned their rights to the employer before filing (Assignment signed 1996-07-10 to 1996-07-17, recorded 1996-07-18, reel 008111/0113) — a standard pre-filing employment assignment, not a post-issuance departure. I found no evidence of either inventor leaving the Siemens organization within 12 months of filing, and no assignor-side "fire-sale precursor" pattern. Note: I cannot verify inventor employment status beyond the patent record itself (e.g., SEC/press), so treat "no departure" as not found rather than confirmed absent.

Original assignee

Siemens Rolm Communications, Inc. (Santa Clara, California) — the entity named on the initial, recording-date assignment (reel 008111/0113). The granted patent's face and Google Patents list the original assignee as Siemens Business Communication Systems, Inc., which took title from Siemens Rolm by assignment effective 1996-10-01 (reel 008980/0507).

• Primary line of business: enterprise telecommunications equipment — PBX / digital telephone station sets. This is exactly the "digital communication processing" environment the patent describes (switching network 42 = PBX; digital telephones 44–48).
• Did they ship a product embodying the claims? Likely yes, but not documented in the record I retrieved. The claims describe a converter pair that lets an off-premises digital feature phone reach a PBX over analog lines. Siemens Rolm / Siemens Business Communication Systems sold the ROLM 9751 / Hicom private-switching platforms and digital feature sets (Optiset, ROLMphone) of the same era. I could not, within my search budget, retrieve a product datasheet, manual, or SEC filing tying a shipping Siemens product to the converter architecture of claims 1/5/10 — so I mark this unverified rather than asserting it.
• Current status of the original assignee: no longer exists as a standalone entity. The chain shows it merged into Siemens Information and Communication Networks (eff. 1998-09-30) → Siemens Communications (eff. 2004-09-22) → Siemens Enterprise Communications (eff. 2010-03-04). The enterprise business was ultimately rebranded/spun as Unify, now part of Atos SE. No bankruptcy appears anywhere in the record.

Assignment timeline

Source caveat: the reel/frame numbers, conveyance types, assignors/assignees, recording dates and effective dates below are taken from the assignment abstract of title reproduced in the patent's own Google Patents legal-events section (which mirrors the USPTO Assignment Center record). The "Correspondent" field (attorney/firm of record) was NOT retrievable with the tools available to me — Google Patents does not expose it, and my searches for the underlying reel PDFs returned no correspondent lines for these reels. I am therefore not reporting correspondent names for this patent, because guessing them would be fabrication. See the "Repeat correspondent" signal below for what this does and does not let me conclude.

1996-07-10 → 1996-07-17 (executed) / recorded 1996-07-18 — Reel 008111/0113

• Conveyance: Assignment of assignors' interest
• Assignor: Shaffer, Shmuel; Beyda, William J.
• Assignee: Siemens Rolm Communications, Inc. (California)
• Correspondent: not retrievable — see caveat above
• Context: inventor-to-employer assignment, recorded on the filing date; standard employment capture of title.

1996-07-10 → 1996-07-17 (executed) / recorded 1998-02-09 — Reel 008981/0261

• Conveyance: Assignment of assignors' interest (re-recording/confirmatory of the same inventor→Siemens Rolm conveyance)
• Assignor: Shaffer, Shmuel; Beyda, William J.
• Assignee: Siemens Rolm Communications, Inc. (California)
• Correspondent: not retrievable
• Context: duplicate/confirmatory recording; same signing dates as reel 008111/0113, re-recorded ~19 months later. No change in beneficial ownership.

Effective 1996-10-01 / recorded 1998-02-09 — Reel 008980/0507

• Conveyance: Assignment (intra-group)
• Assignor: Siemens Rolm Communications, Inc.
• Assignee: Siemens Business Communication Systems, Inc. (California)
• Correspondent: not retrievable
• Context: internal corporate reorganization — this is the link that makes Siemens Business Communication Systems the "original assignee of record" on the granted patent.

Effective 1998-09-30 / recorded 2010-04-19 — Reel 024244/0679

• Conveyance: Merger
• Assignor: Siemens Business Communication Systems, Inc.
• Assignee: Siemens Information and Communication Networks, Inc.
• Correspondent: not retrievable
• Context: internal merger/reorg. Note the ~11.5-year gap between effective date and recording date.

Effective 2004-09-22 / recorded 2010-04-22 — Reel 024263/0817

• Conveyance: Merger
• Assignor: Siemens Information and Communication Networks, Inc.
• Assignee: Siemens Communications, Inc. (Florida)
• Correspondent: not retrievable
• Context: internal merger/reorg, again recorded ~5.5 years after the effective date.

Effective 2010-03-04 / recorded 2010-04-27 — Reel 024294/0040

• Conveyance: Assignment
• Assignor: Siemens Communications, Inc.
• Assignee: Siemens Enterprise Communications, Inc. (Florida)
• Correspondent: not retrievable
• Context: internal corporate reorganization — the carve-out/transfer of the enterprise business into the Siemens Enterprise Communications entity.

Effective 2010-11-09 / recorded 2010-11-10 — Reel 025339/0904

• Conveyance: Grant of Security Interest in U.S. Patents (security agreement)
• Assignor (grantor): Siemens Enterprise Communications, Inc.
• Assignee (secured party): Wells Fargo Trust Corporation Limited, as Security Agent (United Kingdom)
• Correspondent: not retrievable
• Context: securitization — the patent was pledged as collateral in a financing, likely tied to the enterprise-business carve-out/private-equity transaction. Not a transfer of beneficial ownership.

Effective 2015-04-09 / recorded 2015-08-24 — Reel 036434/0247

• Conveyance: Assignment
• Assignor: Unify Inc. (Florida)
• Assignee: Unify GmbH & Co. KG (Germany)
• Correspondent: not retrievable
• Context: internal reorg / consolidation into the German parent.

Effective 2014-09-29 / recorded 2015-09-09 — Reel 036574/0383

• Conveyance: Termination and Release of Security Interest in Patents
• Assignor (releasing party): Wells Fargo Trust Corporation Limited
• Assignee (released party): Unify, Inc. (Florida)
• Correspondent: not retrievable
• Context: release of the 2010 security interest (reel 025339/0904) — the collateral was freed. Effective date predates recording by ~11 months.

Effective 2014-03-27 / recorded 2015-10-28 — Reel 037008/0751

• Conveyance: Demerger
• Assignor: Unify GmbH & Co. KG (Germany)
• Assignee: Enterprise Technologies S.a.r.l. & Co. KG (Germany)
• Correspondent: not retrievable
• Context: internal demerger — spinning the IP into a dedicated entity. Effective ~19 months before recording.

Effective 2014-11-18 / recorded 2015-10-28 — Reel 036987/0803

• Conveyance: Assignment
• Assignor: Enterprise Technologies S.a.r.l. & Co. KG (Germany)
• Assignee: Enterprise Systems Technologies S.a.r.l. (Luxembourg)
• Correspondent: not retrievable
• Context: internal reorg — final hop to the Luxembourg holding entity that databases list as current assignee (parent company Atos SE).

Apparent gap I want to flag (not an inference): the chain shows Unify, Inc. as the releasing party in reel 036574/0383 and as assignor in reel 036434/0247, but this patent's abstract of title contains no recorded Change of Name converting Siemens Enterprise Communications, Inc. into Unify, Inc. That rename was presumably accomplished by a separate recording (possibly tied to a different reel/portfolio-wide filing) that is not surfaced on this patent's record. This is a records gap to verify, not evidence of anything improper.

Timeline diagram

timeline
    title Recorded ownership chain of US 5761294
    1996 : Inventors assign to Siemens Rolm
    1998 : Inventor assignment re-recorded
         : Assigned to Siemens Business Communication Systems
    2010 : Merger to Siemens Information and Communication Networks
         : Merger to Siemens Communications
         : Assigned to Siemens Enterprise Communications
         : Security interest to Wells Fargo Trust
    2015 : Assigned to Unify GmbH and Co KG
         : Security interest released
         : Demerger to Enterprise Technologies SARL and Co KG
         : Assigned to Enterprise Systems Technologies SARL
    2016 : Patent term expired

NPE / troll-pattern signals

1. Shell-entity transfer — NOT PRESENT.
Every assignee in the chain is a named Siemens / Unify / Atos group entity, and each transfer is characterized in the record itself as a Merger (024244/0679; 024263/0817), an intra-group Assignment (008980/0507; 024294/0040; 036434/0247), a Demerger (037008/0751), or a corporate assignment (036987/0803). No assignee carries an "IP / Patents / Licensing / Holdings / Ventures" suffix; none is a Delaware/Texas single-purpose LLC; no registered-agent-service address appears. The terminal entities (Enterprise Systems Technologies S.à r.l. / Unify GmbH & Co. KG) sit under Atos SE, a publicly traded operating IT-services and enterprise-communications vendor — not an anonymous shell.

2. Known asserter in the chain — NOT PRESENT.
No assignee matches the referenced NPE lists (Acacia, Marathon, Intellectual Ventures, IPNav, Wi-LAN, Mosaid/Conversant, Vringo, Pendrell, Innovatio, MPHJ, Lumen View, Round Rock, Erich Spangenberg entities). The chain is entirely Siemens → Unify → Enterprise Systems Technologies, none of which appear on the public asserter directories named in the task. Confidence caveat: I could not complete a live query of the Unified Patents / RPX asserter directories for every named entity within my search budget; the Unified Patents patent page for US-5761294 returned only bibliographic data, with no litigation or assertion listing.

3. Repeat correspondent across the chain — UNCLEAR / NOT DETERMINABLE.
This is the one signal I could not evaluate, and I will not guess at it. The correspondent-of-record field (attorney/firm, address) is not exposed by Google Patents and I could not retrieve the underlying reel PDFs for reels 008111/0113, 008980/0507, 008981/0261, 024244/0679, 024263/0817, 024294/0040, 025339/0904, 036434/0247, 036574/0383, 036987/0803 or 037008/0751. Because the recurrence test requires at least two confirmed appearances of the same attorney, I have no basis to mark this present or absent. To close it, pull each reel/frame at the USPTO Assignment Center or via the Assignment API and capture the "Correspondent" block. Given that the chain is a series of ordinary corporate-law merger/demerger filings rather than serial NPE recordings, I would expect a small set of in-house or outside corporate-securities counsel to recur — but that is an expectation, not a finding.

4. Cascading transfers — NOT PRESENT (as an NPE signal).
There is a cluster of four recordings in ~2.5 months in 2015 (036434/0247 on 08-24; 036574/0383 on 09-09; 036987/0803 and 037008/0751 both on 10-28). Taking the event text at face value, however, these are a security-interest release, a consolidation into the German parent, a demerger, and a holding-company assignment — i.e., one coordinated corporate restructuring — not a hop-scotch through unaffiliated LLCs. Two of the four also carry effective dates 11–19 months earlier (2014-03-27 and 2014-11-18), consistent with a single 2014 group reorganization being papered late. No common-principal shell chain; no shared anonymous correspondent address is establishable from the record. Marked not present on the NPE reading, with the late recording noted as a records-quality observation only.

5. Pre-litigation transfer — NOT PRESENT.
No infringement suit naming US 5,761,294 was found (consistent with the earlier litigation section of this analysis), so there is no "first suit" to measure the 6-month window against. The nearest-in-time transfer (reel 036987/0803, effective 2014-11-18 / recorded 2015-10-28) has no associated litigation.

6. Bankruptcy fire-sale — NOT PRESENT.
Nothing in the record indicates a Chapter 7/11 proceeding for Siemens Rolm, Siemens Business Communication Systems, Siemens Communications, Siemens Enterprise Communications, Unify, or Enterprise Systems Technologies. The 2010 Wells Fargo security interest (reel 025339/0904) was a financing pledge, and it was affirmatively released in 2015 (reel 036574/0383) — no foreclosure, no distressed sale.

7. Privateering — NOT PRESENT.
No evidence that an operating company transferred the patent to an NPE to assert on its behalf. The terminal holders are group affiliates of the original corporate family, not third-party assertion vehicles.

8. Defensive aggregator — NOT PRESENT.
The chain does not terminate at RPX, AST, LOT Network, Unified Patents, or OIN. It terminates at Enterprise Systems Technologies S.à r.l. (Luxembourg), under Unify GmbH & Co. KG / Atos SE.

Verdict

Defensive / non-asserting — with an explicit category caveat.

Justification: the entire chain is a single corporate family, not a sale into an assertion vehicle. Title runs from the inventors through reels 008111/0113 and 008980/0507 into Siemens Business Communication Systems, then purely by merger and intra-group assignment (024244/0679, 024263/0817, 024294/0040, 036434/0247, 036987/0803, and the demerger at 037008/0751), ending at an operating enterprise-communications vendor group (Unify / Atos SE). The only security interest (025339/0904) was released (036574/0383), and the patent expired on 2016-07-18 ("Expired – Lifetime") with no litigation ever found. Caveat on the label: the chain does not terminate at a classic defensive aggregator like RPX/AST; "Defensive / non-asserting" is the closest fit because there is zero evidence of assertion and the holder is an operating company whose rights lapsed. Signal #3 (repeat correspondent) remains unresolved because the correspondent-of-record field was not retrievable, so a full clean bill of health is not possible from this record alone.

Verification links:

• USPTO Assignment Center (search by patent number 5761294): https://assignmentcenter.uspto.gov/
• USPTO Patent Assignment Search (same records, legacy UI): https://assignment.uspto.gov/patent/index.html
• Google Patents legal-events / abstract of title used above: https://patents.google.com/patent/US5761294/en

Recommended next step to close the one open field: open each reel/frame listed above in the Assignment Center and transcribe the Correspondent block (attorney name, firm, address) for reels 008111/0113, 008980/0507, 008981/0261, 024244/0679, 024263/0817, 024294/0040, 025339/0904, 036434/0247, 036574/0383, 036987/0803, 037008/0751, then test for recurrence. That is the only way to move signal #3 off "unclear."

Contradiction flag: the previously generated bibliographic section cites the original assignee as Siemens Business Communication Systems, Inc. while the underlying reel 008111/0113 names Siemens Rolm Communications, Inc. as first assignee (with Siemens Business Communication Systems taking title by the 1996-10-01 assignment at reel 008980/0507). Both statements are correct at different points in the chain — this is a clarification, not a conflict — but the bibliographic table's single "Original assignee" cell collapses a two-step 1996 transfer.

I'll research the prior art references cited against US 5,761,294. Let me run parallel searches for details on each cited reference.

Let me search for the remaining two cited references.

Prior-Art Analysis — US 5,761,294 (Shaffer & Beyda)

Patent under analysis: US 5,761,294 — "Method and system for connecting a digital phone limited to analog transmissions" — App. 08/683,441, filed 1996-07-18, issued 1998-06-02. Source: https://patents.google.com/patent/US5761294/en

Scope note / disclaimer: I confirmed the patent identifier I was asked for (5,761,294 / US5761294A / US 08/683,441) against the USPTO-derived record set embedded in the provided full text (Google Patents mirrors the USPTO file). Per your strict rule, I did not substitute any similar number (e.g., I excluded 5,761,295, 5,761,294-family-lookalikes, and Velos/Uniloc/MCOM numbers that surfaced incidentally). Where the open record was thin — specifically US 5,502,727 — I say so rather than inventing detail. This builds on, and does not repeat, the bibliographic/summary sections already generated.

1. The prior-art set to be analyzed

The face of US 5,761,294 lists six references cited by the examiner ("Patent Citations (6)"). All six bear the "Cited by examiner" asterisk in the record:

The patent also lists four family-member citations ("Family Cites Families (4)"): US 4,884,269 (Hayes Microcomputer Products — "Method and apparatus for connecting ISDN devices over an analog telephone line," 1988-06-20); JP 2645602 B2 (Fujitsu — "Analog trunk line-ISDN converter," priority 1990-02-27); BE 1007360 A3 (Atea NV); and GB 9404981 D0 (Møller). These are the same-family/associative citations and are secondary to the six examiner citations.

Bottom line up front: None of the six references, taken alone, discloses the combination that defines the independent claims — i.e., a converter pair at both ends, no digital lines, voice converted onto the PSTN while the party-specific and call-handling digital-phone messages are converted onto a packet data network, then rejoined and reconverted. Each reference reads cleanly on one slice of a claim (generally the conversion stage of claim 1/5/10), which is why the examiner cited them; but each is properly an obviousness (§ 103) reference, not a standalone § 102 anticipator. I flag below where a § 102 argument is at least arguable.

2. Reference-by-reference analysis

A1 — US 4,998,274 (Bell Communications Research) — "ISDN dual bus extension phone system"

• Full citation: US 4,998,274, "ISDN dual bus extension phone system," inventor Robert M. Ephraim, assignee Bell Communications Research, Inc.; App. 07/446,639 filed 1989-12-06; issued 1991-03-05. https://patents.google.com/patent/US4998274
• Brief description: Adds an AC-coupled analog-side interface to a conventional ISDN terminal so that analog phones (and other ISDN sets) can share a premises "analog bus" as extension phones. The terminal's digital section (Q.931 D-channel call control, B/D-channel mux/demux, codec) is left intact; the invention taps the analog side of the codec. It expressly discusses D-channel "Q.931 call control messaging," CONNECT messages, and off-hook/current-flow sensing (LED 33 local indication).
• Claims it potentially anticipates (§ 102): Weakly, claim 3 (imposing a selected protocol on converted signals) and the converting step of claim 1 (digital↔analog conversion of an ISDN set's channels inside a "converter"). It also touches the type of message traffic recited in claim 4/10 (call-handling messages such as CONNECT). It does not anticipate claim 1, 5, or 10 as a whole: the analog bus is premises-local, not a PSTN/data-network transport, and there is no remote site, no data network, and no split voice-vs-message transport.
• Analyst note: Best characterized as 102(b) art only for its isolated "ISDN-to-analog-tap" teaching; anticipatorily insufficient alone.

A2 — US 5,305,377 (Sun Microsystems) — "Apparatus for providing an ISDN to analog interface"

• Full citation: US 5,305,377, "Apparatus for providing an ISDN to analog interface," assignee Sun Microsystems, Inc.; filed 1991-03-29; issued 1994-04-19 (counterpart EP 0 506 276 A2). https://patents.google.com/patent/US5305377
• Brief description: An interface that separates ISDN signals into control and data signals (D-channel control at 16 kbit/s; B-channel data at 64 kbit/s), converts digital ISDN to analog telephone signals and back, and generates DTMF/ring/on-hook/off-hook tone control. It names typical control content — "the number being called, a ring signal, on-hook, busy, bad number, connected, type of call (voice, data), and off-hook signals." It is bidirectional (analog→digital in the reverse direction).
• Claims it potentially anticipates (§ 102): This is the strongest § 102 read for the conversion limitations: it maps onto the "converting outgoing digital signals … into a voice analog signal … and a message analog signal" step of claim 1 and the "conversion means … for converting telephone digital signals into outgoing analog voice signals and outgoing analog message signals" element of claim 5, and onto claim 3 ("imposing a selected protocol") and claim 7 ("protocol converter"). The "connected / type of call" content also touches claim 4/10 call-handling messages. But it stops at a point-to-point ISDN-to-analog interface — no PSTN-vs-data-network split, no remote switching site, no rejoining of separated streams. So it cannot anticipate the full scope of claims 1, 5, or 10.
• Analyst note: Because it discloses "separating ISDN control from data," an examiner could pair A2 with a network reference (e.g., A6) to attack claim 1/5/10 under § 103.

A3 — US 5,412,709 (Charter Leasing) — "Digital telephone station line controller"

• Full citation: US 5,412,709, "Digital telephone station line controller," assignee Charter Leasing Corporation; App. filed 1992-06-24; issued 1995-05-02. https://patents.google.com/patent/US5412709
• Brief description: A station-line controller connected via a multiplexed digital channel (T1 span) to the line side of a central office of the PSTN and via digital station loops to digital telephones. It monitors terminal devices, reports status to the PSTN in response to status requests (interpreting TR8-style signals), generates test codes, and transmits/receives non-voice information over a subscriber channel during the on-hook, no-ring state (i.e., signaling bits in a digital subscriber channel).
• Claims it potentially anticipates (§ 102): Its subject matter (digital telephones ↔ a switching office) and the carriage of non-voice/signaling information alongside digital voice toward/from the PSTN bear on claim 5 (system of digital telephone + switching support) and claim 10 (call-handling-signal support for a digital set). However, A3's whole premise is that no digital-to-analog conversion is needed ("without any digital/analog conversion") — the opposite of the '294 invention, whose purpose is to make an analog-only link work. It therefore can only be a § 103 background reference; it has no reasonable § 102 anticipation of any independent claim.

A4 — US 5,440,613 (AT&T) — "Architecture for a cellular wireless telecommunication system"

• Full citation: US 5,440,613, "Architecture for a cellular wireless telecommunication system," inventor James Joseph Fuentes, assignee AT&T Corp.; App. filed 1992-12-30 (continuation of Ser. 07/998,310, filed 1992-12-30); issued 1995-08-08. https://patents.google.com/patent/US5440613
• Brief description: A PBX is connected to cellular cell sites over primary-rate-interface ISDN links (multiple B-channels for voice + D-channel for signaling) through a protocol converter that translates between ISDN and the cell-site protocol. The PBX sets up cellular calls as if the cell sites were other ISDN stations; roaming/handoff handled via X.25 messages. FIG. 4 (the wireless/handoff figure) and the PSTN connection are notable.
• Claims it potentially anticipates (§ 102): Directly relevant to the protocol-converter / conversion-means elements: claim 3 ("imposing a selected protocol"), claim 5 ("conversion means … protocol converter" and claim 7), and the "locally supporting a digital protocol" preamble of claim 10. It is also the natural § 103 support for the wireless/cellular analog embodiment of FIG. 4 described in the '294 specification. It does not disclose the claim-1 split of voice→PSTN and messages→a packet data network with reconversion at a remote site; the signaling (D-channel/X.25) travels within the same ISDN/cellular infrastructure. No standalone § 102 anticipation of claims 1, 5, or 10.

A5 — US 5,502,727 (AT&T) — "Image and audio communication system having graphical annotation capability"

• Full citation: US 5,502,727, "Image and audio communication system having graphical annotation capability," assignee AT&T Corp.; filed 1993-04-20; issued 1996-03-26.
• Brief description (flagged for lower confidence): A multimedia communication system combining image, audio, and graphical-annotation capability at a workstation/terminal — i.e., a visual display used to present information correlated with a communication session. The open-web record I could retrieve did not return the full specification before my search budget was exhausted, so this description is drawn from the title, assignee, and class and should be treated as provisional; verify against the printed patent (USPTO Patent Public Search / Patent Center) before relying on it.
• Claims it potentially anticipates (§ 102): By its nature (a visual display presenting session information), the plausible mapping is to the display limitation of claim 9 ("a visual display … responsive to user-specific call messages that are transmitted with call-handling call messages within said analog message signals") and the display screen 62 / light indicators 64 of the '294 specification. It has no teaching of the analog-transport/converter architecture of claims 1, 5, or 10 and is at most a § 103 secondary reference for the display feature. Do not treat A5 as an anticipator without reading the full text.

A6 — US 5,459,780 (AT&T IPM) — "Work at home agent ACD call distribution"

• Full citation: US 5,459,780, "Work at home agent ACD call distribution," inventor Bernard S. Sand (per the granted front page), assignee AT&T IPM Corp.; App. filed 1994-01-31; issued 1995-10-17. Related to Mary R. Otto's "Work At Home ACD Agent Network," US 5,392,345. https://patents.google.com/patent/US5392345 (related) / https://uspto.report/patent/grant/[5459780](/patent/5459780)
• Brief description: A remote (work-at-home) agent is connected to the ACD home switch by (i) an audio path established over the public switched telephone network (PSTN) and (ii) a separate data connection to a Home Agent Server (HAS) that carries event/application messages. The patent expressly states the audio path "is not part of an integrated voice/data path," and that the audio connection and the data path are separate and independent; call event messages (answer, disconnect) flow on the data link while voice flows on the PSTN. Claim 1 recites establishing the audio connection "via a public switched telephone network … wherein said audio path is not part of an integrated voice/data path."
• Claims it potentially anticipates (§ 102): This is the closest prior art to the '294 concept of "split transport." It discloses, for a telephone call, voice over the PSTN on one path and call-related messages over a data network on a separate path, with the two coordinated at a server. It therefore reads on the architecture of claim 1 (voice analog signal via public telephone switching network; message signal via a data network) and the parallel system limitations of claim 5. On a strict § 102 view, however, A6 does not disclose: (a) a converter that locally supports the digital phone's native protocol; (b) conversion of the digital telephone's own party-specific and call-handling messages (A6's data link carries ACD event/application data, not the phone's digital feature-signaling); or (c) rejoining and reconverting the separated streams into a digital protocol for a remote switching network. So even A6 is best used in § 103 combination (e.g., A6 + A2 or A6 + A4) rather than as a clean § 102 anticipator.
• Analyst note: This is the reference an examiner would most likely have leaned on for the "separate voice and message paths" idea, and it is the one I would prioritize in any invalidity/validity analysis.

3. Mapping summary — which reference bears on which claim

4. Overall § 102 / § 103 conclusion

1. No single cited reference anticipates independent claim 1, 5, or 10. The shared novelty that defeats a § 102 attack is the specific conjunction of: (i) a converter that locally supports the digital telephone's own protocol, (ii) conversion of that phone's voice and its party-specific/call-handling call messages into separate analog signals, (iii) transport of voice via a public telephone switching network and messages via a data network, and (iv) reconversion/rejoin into the digital protocol at the remote site — all "without digital lines" (claim 1's express premise).
2. The references are § 103 material. The strongest obviousness combination suggested by the record is A6 (US 5,459,780 — separate PSTN voice path + separate data path) in view of A2 (US 5,305,377 — ISDN control/data separation and D/A conversion) and/or A4 (US 5,440,613 — protocol converter), optionally with A1 for the ISDN-to-analog tap and A5 for the display. That is the combination a Petitioner or examiner would build; it is not a § 102 showing on any single reference.
3. Confidence and caveats: The citations, dates, titles, and assignees for A1–A4 and A6 are corroborated by the granted texts and multiple mirrors. A5 (US 5,502,727) is the one entry I could not fully verify within budget — treat its description and claim-9 mapping as provisional and confirm against the printed patent. Also note the record contains at least one internal inconsistency in the family data (the '294 front matter and the related Otto patent US 5,392,345 appear in citation chains), and the A6 front-page inventor name (Sand) versus the family's Otto patent (US 5,392,345) — these are as printed and are not auto-corrected here.

If you want, I can next: (a) pull the full text of US 5,502,727 to firm up the claim-9 mapping, or (b) build a formal element-by-element § 103 chart for claim 1 against A6+A2.

I'll research the cited prior art references to ground the obviousness analysis.

Let me research the remaining references, including the family citations.

Obviousness Analysis — US 5,761,294 under 35 U.S.C. § 103

Scope note. This builds on the claim-mapping and bibliographic work already produced above; I do not repeat it. It uses the prior-art set appearing in the "Patent Citations (6)" and "Family Cites Families (4)" sections of this patent's record. Reference numbers and titles are quoted literally; where I could not verify internal disclosure detail, I say so rather than inventing pin cites. This is a technical invalidity analysis, not a legal opinion — and it is academic here, since the patent is "Expired – Lifetime" (anticipated expiration 2016‑07‑18) with no litigation identified.

1. Legal framework and the POSITA

Governing test (pre-AIA § 103(a), Graham v. John Deere): scope and content of the prior art; differences between the prior art and the claims; level of ordinary skill; secondary considerations. KSR Int'l v. Teleflex adds that a combination of familiar elements according to known methods is obvious when it yields predictable results, and where there is a "finite number of identified, predictable solutions," pursuing them is "obvious to try." A reference need not address the same problem as the invention (KSR; In re Kahn); it is enough that it comes from the same field of endeavor or a field of reasonably pertinent problem-solving.

POSITA (as of the 1996‑07‑18 priority date): a B.S. in electrical/computer engineering or equivalent, with ~2 years' experience in PBX/telephony or data networking. I note that this skill level is essentially the one a PTAB petitioner stipulated in a parallel-era petition surfaced in my searches ("Bachelor of Science degree in Electrical Engineering, Computer Engineering, Computer Science... and two years of work experience in telecommunications or networking"), which is consistent with this art.

Field: both references relied on below sit squarely in the "connect digital telephone/ISDN equipment to analog links" and "extend PBX/ACD service to off-premises stations" arts — the same field as the '294 patent.

2. The prior-art references, mapped to what they actually disclose

Caution (already flagged in the litigation section, repeated because it matters here): the "Cited By (69)" list is prosecution/patenting aftermath, not prior art. In particular WO 1998/042104 (VocalTec) has a 1997‑03‑16 priority date — after '294 — so it is not available as § 102/103 art against '294 and must not be used as such.

3. What the claims require, and where the gaps are

Because this section builds on the earlier claim overview, I compress:

The only genuinely distinguishing architectural feature of independent claims 1/5/10 is: voice on the circuit-switched public telephone network, call messages on a packet data network, with converters at both ends that speak the digital set's protocol. Everything else (protocol conversion, modem transport of signalling, analog bridging, two-wire analog extension, display of feature data) is expressly in the cited art.

4. Proposed grounds of obviousness

Ground 1 — Claims 1–4, 10–12 obvious over US 4,884,269 (Hayes) in view of US 5,459,780 (Sand)

Hayes supplies nearly the whole claim skeleton, minus the second network. Hayes teaches: (i) a protocol converter at each end that locally supports the digital (ISDN) device's protocol and imposes the correct NT/TE role — claim 3, claim 7, and the "locally supporting a digital protocol" limitation of claim 10; (ii) extraction of D-channel signalling, its modem transport over an analog line, and its reinsertion into the digital stream at the far end — the "message analog signal" conversion and inter-end reconversion steps (claim 1 steps; claim 2); (iii) buffering/packetizing/flow-control of the signalling stream — the claim-4/claim-10 message carriage and the claim-6 modem.

Sand supplies precisely the missing element — which network carries the messages — and, critically, the motivation. Sand connects a remote station to the serving switch by a PSTN audio path plus a separate data connection over "any available data network" for call-event messages, and expressly recites (and claims) that the audio path is "not part of an integrated voice/data path." Its background identifies the reason: integrated digital (ISDN) voice/data facilities were "expensive and only selectively available." That is exactly the problem the '294 patent's own "Description of the Related Art" recites ("availability and cost... digital links are not universally available... such links may be significantly more expensive than analog links").

Motivation to combine: Sand is not merely analogous art; it is the same solution to the same problem. A POSITA seeking to give an off-premises user a full-featured digital set (the '294 goal) would (a) take Hayes's protocol-converter/modem pairing to make the digital set work over an analog loop, and (b) route the signalling/message stream over the data network per Sand rather than sharing the voice line, because Sand teaches that this avoids the expensive/unavailable integrated digital path and that the signalling path need not be time-synchronized with voice (Sand's event messages are latency-tolerant, as the '294 specification itself concedes: "The call messages are not time-critical"). Predictable result, no new technology, finite solutions → KSR.

Claim 10's "party-specific" messages: claim 10 (and claim 4) require feature-phone/display messages on the data path. Sun (below) and Hayes establish that the D-channel message set carried over the analog path already includes call-control information ("connected," "type of call," ring, busy). Party-specific information such as calling-party number, forwarding origin/reason, and message-waiting status are simply further Q.931 information elements on the same signalling channel; sending them over the same message path is a distinction of content, not architecture. US 5,412,709 reinforces this by expressly teaching that non-voice information on a subscriber channel is decoded and output to the terminal device or auxiliary equipment, and US 4,998,274 shows a terminal-state indicator (LED 33) driven from signalling. Combined, claims 4, 9 and 10's "party-specific" limitation is met.

Claim 11/12 (ISP/Internet; public or private data network): Sand names dial-up public data networks (CompuNet, Telenet) and states the connection may be by "a modem." Selecting the Internet/ISP as the data network and characterising it as public or private is the routine use of a known medium, nothing more.

Ground 2 — Claims 1–4, 10–12 obvious over US 5,305,377 (Sun) in view of US 5,459,780 (Sand)

Sun is arguably closer on the split concept than Hayes, because Sun's interface separately manipulates control (D-channel) and data (B-channel) signals and "transfers them along separate paths... alternatively combines them." Its own table of D-channel contents (called number, ring, on-hook, busy, bad number, connected, call type, off-hook) establishes that call-handling messages are conveyed out-of-band from voice, and its architecture provides conversion in both directions with an analog-facing port. Sun thus supplies the "converting outgoing digital signals into a voice analog signal and a message analog signal" step and the reverse-conversion step of claim 1, and the "conversion means... for converting incoming analog voice and message signals into digital signals" of claim 5.

The difference over Sun is again destination: Sun pushes both streams onto the conventional analog telephone line, not onto a data network. Sand supplies that, with the same stated motivation (avoid expensive, selectively available integrated digital facilities; carry call event data on an available data network). Combining Sun's control/data split with Sand's PSTN-audio + data-network signalling produces the claim 1/5/10 architecture with predictable results.

Ground 3 — Claims 5–9 obvious over Hayes or Sun in view of Sand, further in view of US 4,998,274 (Bellcore) and US 5,412,709 (Charter)

• Claim 5 (system with first/second analog lines to conversion means at each of telephone and interface site): Hayes's two interfaces (one NT-side, one TE-side) with analog modem ports teach the "for each of said digital telephone and said interface site, a conversion means" structure; Sand teaches the two separate line classes (PSTN line + data-network line). Bellcore adds analog-extension topology (AC-coupled analog port, analog bus) for the telephone-side analog interface.
• Claim 6 (modem at the telephone-side conversion means on the second analog lines): Hayes's synchronous modem — express.
• Claim 7 (protocol converter): Hayes ("ISDN/RS‑232 interface and protocol converter") and Sun (ISDN↔analog conversion) — express.
• Claim 8 (user data in analog form): Sand transmits application data (order data) over the same data connection; Sun carries a B-channel data stream — express.
• Claim 9 (visual display responsive to user-specific call messages in the analog message signals): US 5,412,709 outputs decoded non-voice information to a terminal device or auxiliary equipment; US 4,998,274 drives a visible indicator (LED 33) from terminal signalling; US 5,502,727 concerns a display-based communication system. Caller-ID-on-a-display was itself well-known consumer art by 1993–96. Confidence: moderate — verified for 5,412,709 and 4,998,274; low on 5,502,727 internals.

Ground 4 — FIG. 4 wireless variant (no separate claim; supporting art)

US 5,440,613 (cellular wireless architecture) is the natural secondary reference for the specification's wireless-analog-link embodiment. Since none of claims 1, 5, 10 requires wireless transmission, this reference is corroborative only. Confidence: low on internals (not retrieved).

Family references (JP 2645602; BE 1007360; GB 9404981 D0): these corroborate the crowdedness of analog↔ISDN conversion and private-telephone emulation but add little beyond Hayes/Sun; I would not rest a ground on them without full text. Confidence: low.

5. Why a POSITA would have been motivated — consolidated

1. Same problem, same field. Hayes, Sun, Bellcore, Charter, and Sand each address connecting digital telephone/ISDN equipment to analog facilities or extending station service off-premises. The '294 patent's stated problem — availability and cost of digital links — is Sand's stated problem almost verbatim.
2. Express motivation in Sand. Sand's background criticizes ISDN-based work-at-home solutions as "expensive and only selectively available" and its claim 1 requires that the audio path "is not part of an integrated voice/data path." This is a direct teaching to route voice over the PSTN and non-voice traffic over a separate data network — the heart of claims 1/5/10.
3. Predictable, known building blocks. Protocol conversion (Hayes, Sun), modem transport of signalling over analog links (Hayes), analog-side tapping/extension of a digital set (Bellcore), conversion means between digital phones and analog line connections (Charter's background), and data-network carriage of call events (Sand). Combining them involves no change in their respective principles of operation.
4. Finite, identified solutions. As of 1996, a designer faced with a digital set and only analog facilities had a small menu: put both streams on the analog loop (Hayes/Sun), or split voice to the PSTN and signalling to a data network (Sand). The latter is "obvious to try" under KSR, and the specification itself concedes the split is beneficial rather than surprising ("the combined benefits of packet-switched technology and circuit-switched technology"; "call messages are not time-critical, so that synchronization... is not required").
5. Reasonable expectation of success. Hayes demonstrated signalling survives analog/modem transport; Sand demonstrated a separate data path adequately carries call events with tolerable latency; the '294 specification confirms non-synchronization is acceptable. No unpredictable result needed.

6. Secondary considerations — no persuasive nexus

• Long-felt need / commercial success: telecommuting "dual presence" was a recognized need, and the Siemens/Unify commercial line suggests success — but any such success is attributable to Siemens' installed PBX base and existing digital set ecosystem, not to a nexus with the claimed two-network split. No evidence of nexus was found.
• No unexpected results: the specification's concessions (Fig. 3's "reduced QoS" and delays on the data network; explicit non-requirement of synchronization) show the results are the expected ones. Lower voice QoS over packet networks is precisely what Sand's "not an integrated voice/data path" architecture was designed around.
• No teaching away. Nothing in Hayes or Sun disparages a separate packet data network; silence is not teaching away (In re Kahn / KSR standards). If anything, Sun's use of the analog line for both streams would steer a designer toward one line, but Sand's express cost/availability rationale overrides that for the off-premises use case.
• Copying/industry praise: not evidenced in the record.

7. Anticipated counterarguments and responses

8. Bottom line

On the record's own prior-art set, the strongest ground is:

• Claims 1–4, 10–12 obvious over US 4,884,269 in view of US 5,459,780 (with US 5,305,377 as an alternative primary reference for the control/data split, and US 5,412,709 / US 4,998,274 for party-specific/display messaging).
• Claims 5–9 obvious over the same combination, with US 4,998,274 and US 5,412,709 for the analog-extension and display/feature-message limitations.
• US 5,440,613 is corroborative art for the unclaimed wireless embodiment; the family references (JP 2645602, BE 1007360, GB 9404981 D0) indicate a crowded analog↔ISDN conversion field but should not carry a ground without full text.

The single inventive-looking feature — voice on the circuit-switched public network and signalling on a packet data network, with protocol-converting converters at both ends — is disclosed in substance by Sand (PSTN audio + separate data network for call events, expressly avoiding integrated voice/data) combined with Hayes or Sun (protocol conversion and modem transport of the digital set's signalling over analog facilities). The motivation is express in Sand and echoed in the patent's own background; the result is predictable; there is no evident teaching away and no nexus-backed secondary consideration.

Confidence: High on Hayes, Sun, Bellcore, Charter and Sand, all of which I verified from primary text this session. Low-to-moderate on US 5,440,613, US 5,502,727 and the three family citations, whose internal disclosures I could not retrieve — treat my treatment of those four as provisional. Also note the two typographical defects carried in the printed claims ("outdoing" in claim 1; "form" for "from" in claim 10), which I have not auto-corrected and which are relevant to claim construction if these claims were ever tested.