Phone line and hand-off architecture

How we host a property's phone number and how Clara hands a caller off to a person — what we run today, what the vendors actually support, and the recommended path. Written for the Western Slope RingCentral onboarding and the Camellia missed-hand-off pain.

Proposed — this week vs. next · deep dive added 2026-09-08

The problem in one paragraph

A missed hand-off is a dead end for the team today. Clara bridges a caller to a person using ElevenLabs' built-in transfer tool, which is a black box: we can't choose what the team member's phone shows, we get no signal when nobody picks up, and the team member's phone log just shows a missed call from Clara's number — not the caller's.

Unless the team member opens the follow-up email Clara sends afterward, that call is simply lost. This is the single most-reported problem across AI-receptionist setups like ours, including two RingCentral community threads describing our exact configuration: a third-party AI running on Twilio, handing calls back into RingCentral.

Deep dive for 2026-09-08: how the market does it, and the vendor tools we are not using

Ask from Fede: "This is an old problem, done many times. Don't reinvent the wheel. If Twilio and ElevenLabs have tools that work reliably and we're not using them, I want to know." Target experience: everything recorded including the hand-off leg; the team phone shows the original caller; if the hand-off lands in voicemail or nobody answers, Clara pulls the caller back and offers to take a message; callers who are used to extensions can still reach a person. All claims below are sourced; anything unsourced is marked as such.

1. The four target behaviors are named, standard features across the market

Behavior we want	Who documents it	What they call it
Team phone shows the original caller on hand-off	Retell ("Displayed Caller ID: User's Number"), Vapi (callerId = customer number), Synthflow (caller's original number, Twilio-purchased numbers only), My AI Front Desk (admin picks caller's number or the AI's), EliseAI (warm transfer shows the original caller's number to the agent)	Caller ID passthrough, gated behind a setting, works best on Twilio-owned numbers
Detect voicemail or no answer on the human leg, then return to the caller	Synthflow ("Human Detection Timeout" plus six transfer outcomes), Retell (voicemail detection), Bland ("Hold Timeout" with fallback message), Funnel Leasing (after 2 to 3 failed attempts routes to the office line and proactively offers to reconnect), PATLive (returns to a live receptionist after N attempts), EliseAI (scripted line: "No one was available to answer the call. I can take a message for you, or is there anything else I can assist you with directly?")	Transfer fallback
Record the whole call including the hand-off	My AI Front Desk ("entire call, including transferred portion"), Dialpad (segments before and after transfer), EliseAI (full call to portal). Caution: Amazon Connect documents that recording STOPS once an agent leaves an externally transferred call, so this is not a universal baseline	Continuous recording
Reach a person by name, extension, or keypad	Dialpad and RingCentral and Nextiva (dial-by-name and dial-by-extension directories), GoTo ("Dial by Name Directory Routing"), Retell (keypad digits captured by default during the AI conversation), Vapi (native keypad input), Bland (interprets incoming keypad presses)	Directory routing; keypad in parallel with speech

Surprises: the mature human receptionist services (Smith.ai, Ruby, Abby, Nexa, PATLive) do not publicly document what caller ID staff see on a transfer. The property-management AI vendors (AppFolio Realm-X, Knock, Entrata, Colleen, ResMan, Yardi) document almost nothing on any of the four; only EliseAI and Funnel are specific. Keypad-during-AI-conversation is a 2024 to 2025 feature, not settled tech, and most AI receptionist startups dropped the keypad entirely.

2. What best-in-class sounds like when the human does not answer

Clara dials the person. She waits to hear live speech before saying anything to them, so her whisper never plays into a voicemail box (Synthflow's human-detection timeout pattern).
If nobody live picks up within the window, the attempt is marked failed with a reason: timeout, voicemail, busy, connection error.
Clara comes back to the caller: "No one was available to answer the call. I can take a message for you, or is there anything else I can help with directly?" (EliseAI's line.)
The message becomes a structured task with the caller's number, not a raw voicemail (EliseAI task, Funnel guest-card note; our missed-call email already has this shape).
If the same caller was failed before, Clara offers to try again without making them re-explain (Funnel).
Final fallback after repeated failures: the office main line, not a dead end (Funnel, PATLive).
The whole thing is one continuous recording.

3. Extensions: why they exist and what modern desks do with that job

Who uses an extension: returning callers and vendors who want to skip the front door and reach a known person, and staff who put an extension on outbound material so a callback lands on them (Nextiva). Western Slope's tree offers extensions but publishes none, and the probe found 100 and 999 invalid, so today only insiders can use them.
Frequent "0 for operator" presses are treated by the phone vendors as a signal the menu has too much friction, not a feature to preserve (Nextiva).
What replaces the unpublished list, in every serious product: recognize the caller by number and route straight to their person (Smith.ai VIP list skips the receptionist entirely); say the name (Dialpad, RingCentral, Nextiva, GoTo directories); keep extension dialing as a parallel directory for those who know one (Dialpad, Nextiva); accept keypad digits mid-conversation for noisy rooms or numeric input (Retell, Vapi, Bland); keep "0" as an escape that is measured.
How the legacy phone system is kept: the AI sits on the main number only. Direct lines, desk phones, and voicemail underneath are untouched (Ooma, Nextiva). RingCentral documents connecting an AI receptionist to an existing PBX so it routes to an "AI extension". Some platforms register the AI as a plain extension on the customer's PBX (3CX, Yeastar via VoiceInfra; Telnyx SIP Attach), which is the deepest coexistence pattern but needs a PBX that allows it. RingCentral does not offer that to small accounts.
No vendor publishes an extension list to fix this. The direction is recognition plus say-the-name, with the keypad as a fallback.

4. Vendor tools we are not using

Tool	Vendor	What it gives us	Works with our current native setup?
Fallback URL on the phone number	Twilio	If ElevenLabs is unreachable, Twilio runs a simple backup: ring a person or take a voicemail. Five-minute console setting. None of our lines has one.	Yes, it lives on the number, independent of who owns the call. Confirm with Twilio that an ElevenLabs-imported number still honors it.
Keypad input during the conversation ("dtmf_input_settings")	ElevenLabs, added 2026-08-31	Callers can press digits while talking to Clara. Directly serves the extension habit.	Yes. No reliability data yet; test at the Willows.
Blind transfer mode	ElevenLabs	The only transfer mode documented to preserve the original caller's number on the team phone.	Yes, one config flag. Loses the whisper; no no-answer signal.
Voicemail detection system tool	ElevenLabs	Detects a voicemail greeting and can play a message or bail.	Presumed yes. Accuracy not published.
Call token on the dialed leg	Twilio	Carries the original caller's verified identity onto the leg we dial, so the team phone shows the real caller with proper trust signing instead of a low-trust forwarded number.	Only if we run the dial ourselves.
Answering machine detection on the dialed leg	Twilio	94% accuracy, near 100% in message-end mode. The mature way to know a hand-off hit voicemail.	Only if we run the dial ourselves.
Dial action callback with the dial outcome	Twilio	The exact mechanism for "nobody picked up, bring the caller back to Clara."	Only if we run the dial ourselves.
Whisper and "press 1 to accept" on the dialed leg	Twilio	Team member hears who is calling and accepts before the bridge.	Only if we run the dial ourselves.
Dual-channel recording on the dialed leg	Twilio	Guaranteed recording of the human leg.	We already do this at call start; it stays true under our own dial.
Caller name lookup	Twilio	Name, not just number, on the team phone and in the email. US only, small per-lookup fee.	Yes.
Voice Integrity registration	Twilio	Registers our numbers with the carrier spam-analytics programs so our calls stop being labeled.	Yes.
ConversationRelay	Twilio	Twilio-first architecture where Twilio owns the call and uses ElevenLabs voices. Native keypad, hand-off, recording.	No, it is a different architecture. Large rebuild.
Workflows visual builder with transfer nodes	ElevenLabs	Success and failure edges on tool nodes.	Yes, but failure-edge behavior for transfers is not fully documented.

Gaps confirmed on the ElevenLabs side: no documented caller ID behavior for conference transfers, no ring timeout, no "resume after failed transfer," no transfer outcome in the post-call webhook, and an open bug where the transfer cuts Clara off mid-sentence.

5. Three ways to get there, sized

Path	What it gets us	What it cannot do	Size
A. Stay native, configure harder	Fallback URL; keypad input; blind transfer for caller ID; voicemail detection; caller name lookup	No reliable "bring the caller back" on no-answer; no whisper if we go blind; caller ID on conference mode stays undocumented	Small, settings plus a Willows bench
B. Native conversation, our own hand-off through Twilio	Every target behavior: original caller with trust signing, whisper, voicemail detection, return to Clara on no-answer, structured message, continuous recording, keypad and name routing	Needs a custom transfer tool, a small bridge handler, and confirmation that Clara can take the caller back after a failed dial	Medium, the June "own the dial" option; the same shape Retell, Vapi, and Bland implement internally
C. Twilio-first (ConversationRelay or SIP trunk)	Cleanest control	A rebuild of the call layer; ElevenLabs SIP mode has no whisper	Large, not now

6. Proposed staging (for Fede to accept or change)

This week (Western Slope goes live on forwarding):

Fallback URL on every production line, pointing at a human line.
Turn on the missed-call email for Western Slope (already carries the caller's number).
Team directory on the property record: name, role, direct number, old extension. Clara routes by name or spoken extension. Repoint the "talk to a person" bridge off the main number.
Willows bench: blind transfer (does Kat's phone show the caller?) and the new keypad input.

Next two to four weeks, one property at a time:

Path B for the hand-off: our own dial with original caller, whisper, voicemail detection, and return-to-Clara. Bench, soak, Western Slope first, then Camellia.
Recognize known callers by number and route straight to their person (the modern replacement for extensions).
Register numbers with Voice Integrity; add caller name lookup to the email.

Questions to put to support before relying on anything: ElevenLabs, what caller ID does conference transfer present, is there any ring timeout or resume-on-failure, does recording continue after the agent leaves. Twilio, does the fallback URL fire on an ElevenLabs-imported number, and can an agent regain the caller after our own dial reports no-answer.

Sources for this section

Retell transfer · Retell keypad · Retell voicemail
Vapi transfer · Vapi keypad · Vapi forwarding
Synthflow transfers · Synthflow transfer to humans
Bland warm transfer
EliseAI call center guide · EliseAI transfer fallback
Funnel voice
My AI Front Desk transfer · recording
Smith.ai VIP list · Smith.ai transfer by SMS
PATLive transfers
Dialpad directories · Dialpad transfer
RingCentral dial-by-name · RingCentral AI receptionist with PBX
Nextiva extensions · Nextiva auto attendant · Nextiva direct dial · Nextiva on "0"
GoTo AI receptionist · Ooma direct dial
AI as PBX extension · case · Telnyx SIP Attach
Amazon Connect recording caveat
ElevenLabs keypad input changelog · voicemail detection · transfer · post-call webhooks · workflows · status history
Twilio Dial · AMD · AMD FAQ · failover · screening · SHAKEN/STIR · Lookup · ConversationRelay
Western Slope phone tree map (internal): ~/westernslope-data/phone-tree.md, probed 2026-09-05.

What we run today (Camellia, Yale 25 Station, Western Slope test line)

Number hosting: the customer keeps their number and forwards calls to a Twilio number we own. That Twilio number is imported into ElevenLabs, so ElevenLabs owns the call from ring to hang-up. Our only hooks are the "who is calling" webhook at call start and the post-call webhook at the end.
Hand-off: ElevenLabs "conference" transfer, one destination number per property. Clara says "Let me connect you with someone on the team," dials, whispers the caller's name to whoever answers, then drops off.
Caller ID on the team member's phone: not controllable and not documented by ElevenLabs. On the Western Slope test line, our own bridge code explicitly shows a PropFlow number.
No-answer detection: none from ElevenLabs. We reconstruct it after the fact by analyzing the two-channel Twilio recording (connected / voicemail / no pickup / silence). That's why the missed-call email arrives late, and why eleven Camellia hand-offs in the August count died silently.
What the team gets after a miss: one email with the caller's number in it. No text to the team (removed 2026-08-31 at Fede's request). Nothing to the caller.
Recording: yes. We start a two-channel Twilio recording at answer, so both Clara's side and the human side are captured. We keep only the human leg. This works with the conference transfer because the call never leaves Twilio.
Failover: no fallback URL on any production number. If ElevenLabs is down, callers get dead air. ElevenLabs had a 16-hour inbound-routing incident on 2026-07-31.
Recording disclosure: none on inbound agents. Colorado is one-party consent so this is legal, but it's a gap already flagged in our own portfolio architecture notes.

What the vendors actually support (documented)

ElevenLabs transfer to number

Three modes: conference (default, what we use), blind, SIP REFER.
Blind transfer is the only mode documented to preserve the original caller's number. It can't whisper, and Clara can't stay on the line to announce the caller.
No parameter for caller ID, ring timeout, or no-answer behavior. The post-call webhook doesn't report whether a transfer succeeded.
Known bug: the conference transfer can cut Clara off before she finishes her "please hold" line. The field fix is to fire the transfer from your own backend with a 2-to-5-second delay.

Twilio (we own the number and the account)

A bridged call can present the real caller's number as caller ID — this is the standard call-forwarding case and is legal under the Truth in Caller ID Act, since there's no intent to defraud.
Trade-off: a forwarded call gets the lowest STIR/SHAKEN signing level (attestation C with a diversion marker), so carriers may label it "Spam Likely" on the receiving phone. Mitigation: register our numbers in Twilio Voice Integrity, keep complaint rates low, and let each property choose between "show the caller" and "show Clara."
Full control on the human leg: ring timeout, simultaneous ring to several numbers, a whisper to the team member before bridging, answering-machine detection so we never hand a caller to a staffer's voicemail, and a callback the instant the leg ends with busy / no-answer / failed.
Two-channel recording of the whole call including the human leg, and a fallback URL that runs if the primary handler fails.

RingCentral

Forwarding the main number to an outside number passes the original caller's number through — no setting changes it. This is the hop that works today.
Forwarded minutes draw from the plan's outbound minutes, no separate fee.
RingCentral's own AI receptionist has the same documented issue: transfers show the AI's number, not the caller's.

How peers do it

Forwarding the customer's existing number is the market-standard entry path. Porting the number is a later "own it fully" upgrade offered by a few vendors (Smith.ai, My AI Front Desk).
Warm transfer with a spoken summary is the professional norm; blind transfer is the fallback.
Showing the original caller to staff is an explicit, configurable option on Retell and Synthflow, and only works when the AI's number is a Twilio-purchased number — ours are.
Missed transfers everywhere become voicemail plus a notification to staff. Synthflow tracks six transfer outcomes and can retry or fall back. Nobody automatically re-rings.
Property-management-native agents (AppFolio Realm-X, Knock, Entrata) sidestep the caller-ID problem entirely by opening the guest card for the receiving agent instead of relying on the phone's caller ID.

Most-reported pitfalls in the field

Rank	Pitfall	Source
1	Caller ID replaced by the AI's number on the transfer leg	RingCentral forums, Retell community, Canadian carrier reports
2	ElevenLabs conference transfer cutting off early	ElevenLabs / Twilio field fix
3	Voicemail mistaken for a person, so the AI "succeeds" into a voicemail box	Field reports
4	Codec and touch-tone distortion on multi-hop bridges	Field reports
5	Echo and half-duplex audio right after a transfer	Retell community
6	Unowned caller ID blocked or spam-flagged	Twilio spam remediation docs
7	Per-leg cost stacking — Twilio bills each leg, plus a small fee to receive a transferred call	Twilio billing docs
8	Single-vendor outage with no fallback	ElevenLabs status history
9	Ring-timeout and busy-line handling gaps	Field reports
10	Recording consent on the transferred leg in all-party-consent states	Recording-law surveys

Options

Option	What it is	Fixes	Leaves broken / cost
A	Keep everything as is	Nothing — cheapest, zero rework	Dead-end missed calls, late no-answer detection, the cut-off bug
B	Switch ElevenLabs to blind transfer	Caller ID on the team phone shows the real caller	Loses the whisper and Clara's announcement; still no no-answer signal; still nothing on failover. Not enterprise-grade.
C recommended	Own the hand-off on Twilio. Clara's transfer becomes a tool call to our backend, which moves the live call into our own bridge: dial the team member with the caller's real number (or Clara's, per property), whisper who's calling, detect voicemail, ring up to N seconds, and if nobody answers bring the caller back to Clara to take a message, then email the team with the caller's number.	Real no-answer signal instead of after-the-fact recording analysis (a deletion, not an addition); two-channel recording of the human leg comes for free; full control over caller ID, ring timeout, voicemail detection	Rewiring the transfer tool on every specialist agent; a new Twilio bridge handler; a bench run at the Willows; an overnight soak; then rollout one property at a time. Risk of spam labels when presenting the caller's real number — default is Clara's number plus whisper, with "show the caller" as a per-property switch.
D	Twilio-first with ElevenLabs behind a SIP trunk	Maximum control and recording regardless of transfer mode	Bigger rebuild; ElevenLabs SIP mode has no whisper; ElevenLabs SIP routing had the July incident. Not now.

Option C was called "own the dial" in the June design note and was deferred then as too much rewiring.

Recommendation

Decision (Fede, 2026-09-07): this week = forwarding + settings; keep improving toward the full experience

Western Slope this week: forward the main number from RingCentral to Clara's line, staff stay on RingCentral, watch what breaks. No porting.
Turn on the existing missed-call email (it already carries the caller's number) for Western Slope; that alone fixes "can't call them back."
Test ElevenLabs' blind-transfer setting on the "talk to a person" hand-off at the Willows: it is the one documented mode that shows the real caller's number on the team phone. Cost: no whisper.
Add a fallback number on every production Twilio line so an ElevenLabs outage rings a person, not dead air.
Before forwarding goes live, repoint the test line's "talk to a person" bridge away from the customer's main number so it does not loop into Clara. Done 2026-09-08: the bridge is gone from the agent; in-hours "talk to a person" now dials a direct desk number read from the property record (blank today, so Clara takes a message) — #7373; and a failed transfer takes the message instead of reading out their main line — #7374. Live state and the switch-on steps: the onboarding plan, prototype-line lane.
Target experience, staged over the coming weeks: everything recorded, the team phone shows the original caller, and if a hand-off lands in voicemail Clara pulls the caller back and offers to take a message. A deeper look at how front-desk products and our own vendors' tools do this is due 2026-09-08 morning and will be added to this page.

Answers to Fede's direct questions

Does "press extension X" go away?

Yes, once the main number forwards to Clara. Callers say the name; Clara transfers to that person's direct number. Direct lines keep working for anyone who already has them.

What does Kat's phone show today on a hand-off?

A PropFlow number, not the caller's. Under option C it can show the real caller instead, with the spam-label trade-off described above.

Are hand-off calls recorded?

Yes — with Clara at the top we record every call end to end, including the hand-off leg, today and under option C. Only the SIP REFER transfer mode would lose the recording, and we don't use it.

Sources

Internal: docs/planning/unknown-caller-transfer-no-answer-detection.md (June 2026 design, options 2a–2d); agents/clara/lib/voice/post-transfer.ts (recording at answer, outcome classification); config/phone-registry.json; agents/clara/lib/voice-agents/prototypes/western-slope-portfolio-triage.ts (bridge callerId).