does 11 labs use the outbound trunk configuration when doing call transfers or not? | ElevenLabs | Page 1

still mountainBOT Sep 8, 2025, 12:37 PM

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AI-generated Answer for johnnyflaco

Short answer: It depends on the transfer method.

Conference transfer: Yes. ElevenLabs “calls the destination” to add them to the call; outbound calls initiated by ElevenLabs are sent to your SIP provider using your configured outbound trunk/hostname. (elevenlabs.io)
SIP REFER transfer: Not necessarily. REFER is performed over the existing SIP session and requires your SIP trunk to allow REFER; the docs do not state that outbound trunk settings are used for REFER. (elevenlabs.io)

Related constraints: Only SIP-trunked numbers can transfer to external numbers; Twilio-hosted numbers can transfer only to other Twilio numbers unless you use Twilio Elastic SIP Trunking via SIP trunking. (elevenlabs.io)

Sources:
Transfer to human | ElevenLabs Documentation
Transfer to human | ElevenLabs Documentation
Transfer to human | ElevenLabs Documentation

onyx gulch Sep 8, 2025, 1:06 PM

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but my 3cx isnt geting any refers

still mountainBOT Sep 8, 2025, 1:06 PM

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AI-generated Answer for johnnyflaco

You’ll only see a SIP REFER from ElevenLabs if all of the following are true:

The agent’s “Transfer to Human” rule is set to Transfer Type = SIP REFER (the default is Conference, which does not send REFER). (elevenlabs.io)
The live call to the agent is using a SIP trunk (not the Twilio native integration). SIP REFER is only available “when using SIP protocol during the conversation.” (elevenlabs.io)
Your SIP trunk permits transfers via SIP REFER; if the trunk/PBX disallows REFER, none will be sent. (elevenlabs.io)

Related notes:

Only SIP trunk numbers can transfer to external numbers; Twilio numbers can only transfer to Twilio-hosted numbers. If you need external transfers from Twilio, use Twilio Elastic SIP with our SIP trunking instead. (elevenlabs.io)
SIP trunk setup (inbound/outbound trunks, transport, and authentication) is configured under SIP Trunking; ensure your SIP trunk is correctly configured and the call is actually flowing over that trunk. (elevenlabs.io)
The platform now separates inbound_trunk and outbound_trunk in phone number config; transfers support both conference and SIP REFER. (elevenlabs.io)

If your 3CX call meets those conditions and you still don’t see a REFER, please confirm:

The specific call was to a SIP-trunk-imported number (not Twilio native). (elevenlabs.io)
The transfer rule for that agent is explicitly set to SIP REFER. (elevenlabs.io)
The trunk allows REFER on your side (per your PBX/trunk policy). (elevenlabs.io)

Happy to check your agent/number IDs if you share them.

onyx gulch Sep 8, 2025, 1:35 PM

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We are using an AI Agent to place an outbound SIP call to our 3CX Phone System. The goal is for the agent to connect an external user to one of our call queues and then bridge the audio, keeping the call active for the user (a "conference transfer").

Problem:
The agent successfully calls our 3CX system, and our system correctly answers the call by sending a 200 OK SIP response. However, the instant the call is answered, the agent's platform sends a BYE request, terminating its side of the call.

This causes our PBX to drop the entire session because the initiating party has disappeared. The end-user is disconnected before any audio is heard.

Technical Evidence:
Our call logs consistently show this pattern:

INVITE from ElevenLabs Agent
200 OK (Answer) from our PBX
Immediate BYE (Hang up) from the ElevenLabs Agent, typically within 0.1 seconds of the answer.
Our PBX then logs Terminate call as no connected legs remains.

Our Question:
We need the agent to perform a conference transfer, where it stays on the line to bridge the audio between the original user and our PBX.

How can we configure the agent's tool (transfer_to_human or similar) to remain on the call after it's answered and not send an immediate BYE message? We cannot use a REFER transfer for our use case.

still mountainBOT Sep 8, 2025, 1:35 PM

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AI-generated Answer for johnnyflaco

Short answer: This is expected with ElevenLabs’ “Conference Transfer.” The transfer_to_number tool only supports:

Conference Transfer (default): dials the destination, adds them to a conference, then removes the AI agent so only the original caller and the transferee remain. There is no documented option to keep the agent on the line as a third leg. (elevenlabs.io)
SIP REFER Transfer: hands the call off with SIP REFER (requires SIP and a trunk that allows REFER). (elevenlabs.io)

Because Conference Transfer explicitly removes the AI agent, the agent will not remain to bridge audio after the destination answers. The docs do not describe any parameter to change this behavior (e.g., a “stay on call” flag). The only recently noted transfer-related knobs are things like transfer delay/first-message behavior, which do not alter the fact that the agent disconnects in Conference Transfer. (elevenlabs.io)

What you can (and cannot) do per docs:

You can choose between Conference and SIP REFER when defining transfer rules. Conference is default; REFER requires SIP and trunk support. (elevenlabs.io)
The transfer tool is intended to “transfer the ongoing call.” It is not documented to support a persistent three‑party bridge where the AI agent stays connected. (elevenlabs.io)

Given you cannot use REFER, there is no documented way to have the ElevenLabs agent remain on the call after answer for a PBX queue handoff. If you need a true three‑party bridge, the current docs don’t show a supported configuration for that with transfer_to_number. (elevenlabs.io)

References:

Transfer to human (tool behavior and transfer types). (elevenlabs.io)
Changelog note (transfer options like delay/first-message controls do not change agent removal on Conference Transfer). (elevenlabs.io)
Transfer to human (workflows doc; “transfer the ongoing call”). (elevenlabs.io)

onyx gulch Sep 8, 2025, 1:39 PM

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So how is conference transfer supposed to be ussed? Its basically useless then?

still mountainBOT Sep 8, 2025, 1:39 PM

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AI-generated Answer for johnnyflaco

Short answer: Conference transfer is a warm handoff, not a persistent three‑way bridge.

What it does (per docs):

When your agent calls transfer_to_number with transfer_type=conference, ElevenLabs dials the destination and adds them to a temporary conference with the caller, then removes the AI so only the caller and the human destination remain. There’s no documented option to keep the AI on the line after the transfer. (elevenlabs.io)

What it doesn’t do:

Persistent 3‑party conferencing with the AI remaining on the call is not described/supported in the public docs. If you need to transfer to a SIP URI specifically, that requires the SIP REFER transfer type; only REFER supports SIP URIs. (elevenlabs.io)

Routing during transfers:

For SIP trunking, outbound calls (including those initiated by transfers) are sent to the hostname you configured in the phone number’s “outbound settings” (Address). That’s the documented termination target ElevenLabs uses for outbound SIP calls. (elevenlabs.io)

So, conference transfer is intended for warm handoffs to a human/queue and then the AI exits. If your requirement is to keep the AI present after answer, that behavior isn’t available in the published ElevenLabs transfer tooling today. (elevenlabs.io)

Sources:
Transfer to human | ElevenLabs Documentation
Transfer to human | ElevenLabs Documentation
SIP trunking | ElevenLabs Documentation
Transfer to human | ElevenLabs Documentation

onyx gulch Sep 8, 2025, 1:41 PM

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How in 3cx we keep the call active then?

#does 11 labs use the outbound trunk configuration when doing call transfers or not?