#We currently have an issue with the transfer_to_number tool. For some calls, when the user gets tra

1 messages · Page 1 of 1 (latest)

fleet forgeBOT
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AI-generated Answer for p_hinn

Thanks for flagging this. ElevenLabs doesn’t document an “infinite hold music” symptom specifically, but the following documented areas commonly cause transfer_to_number issues. Please verify each:

  1. Confirm you’re using the native Twilio integration (not register_call)
  • The register_call pattern does not support call transfers at all. If any of these calls used register_call, the transfer will not complete. (elevenlabs.io)
  1. Check Twilio regional routing configuration
  • Transfers and other “ongoing call” operations can fail if the Twilio routing region used for the call doesn’t match the region ElevenLabs targets. Specify the correct Twilio region for the number in ElevenLabs; otherwise operations like transfers will fail/misbehave. (elevenlabs.io)
  1. Verify transfer rules and the exact destination format
  • transfer_number must match a number you configured in the tool’s rules, and phone numbers must be in E.164. A mismatch can cause the tool to execute without actually connecting the far end. (elevenlabs.io)
  1. Consider the transfer type and IVRs/queues on the destination
  • Conference transfer (default) dials the destination, joins it to a conference, then removes the AI so only caller and destination remain. If the destination is an IVR/queue that needs DTMF before an agent answers, the caller can be left waiting because the AI is no longer present to press keys. This follows from:
    • Conference transfer removes the AI from the call. (elevenlabs.io)
    • The keypad touch tone tool plays DTMF only while the agent is on the call. (Inference: after removal, the AI can’t send DTMF to advance IVRs.) (elevenlabs.io)
  • If you must traverse an IVR, either send required DTMF before completing the transfer, route to a direct human-answered line, or use SIP REFER (when using SIP) so your telephony platform owns the transfer end-to-end. (elevenlabs.io)
  1. Use workflows to add a failure/success fallback
  • In the visual Workflows builder, a tool node can branch on success vs failure. Implement the Transfer to number node with a failure path (e.g., revert to the agent or end with a message) so callers aren’t stranded if the transfer doesn’t complete. (elevenlabs.io)
  1. Where to look for concrete failure reasons
  • Check the ElevenLabs Call History page; failed calls show error messages (e.g., missing variables after transfer). If no error is shown for a failed transfer, contact Support per the article. (help.elevenlabs.io)

Reference docs for transfer_to_number capability and limits

  • The transfer_to_number tool supports Twilio and SIP trunk numbers; numbers can be external when using Twilio or SIP. Review parameters (client_message/agent_message) and ensure rules are correctly defined. (elevenlabs.io)

If you can share one example with:

  • ElevenLabs call ID, Twilio Call SID(s), transfer type used (conference vs SIP REFER), and confirmation of native Twilio integration vs register_call,
    I can map those details to the specific checks above.
cursive gull
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that means, when we use SIP Refer, we shouldn't have that problem anymore?

fleet forgeBOT
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AI-generated Answer for p_hinn

Not necessarily. ElevenLabs supports two transfer modes for the transfer_to_number tool:

  • Conference transfer (default): dials the destination, places parties in a conference, then removes the AI.
  • SIP REFER transfer: performs a direct SIP REFER handoff to the destination.

SIP REFER removes the conference leg and hands the call off directly, but the documentation does not state it as a guaranteed fix for “waiting music” scenarios. It’s only available when the call with ElevenLabs is over SIP and your SIP trunk allows SIP REFER; you must select “SIP REFER” in the transfer rule. Only SIP REFER supports transferring to a SIP URI, and you can optionally add custom SIP REFER headers. (elevenlabs.io)

If you’re using Twilio phone numbers without SIP trunking, SIP REFER is not available; the feature requires a SIP-based session with ElevenLabs. To use SIP REFER, route calls to ElevenLabs via SIP trunking and ensure your trunk permits REFER. (elevenlabs.io)

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

cursive gull
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But the phonenumber is via Twilio, and when tested SIP-Refer also works? So it should be no issue if we switch it up, no=

fleet forgeBOT
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AI-generated Answer for p_hinn

Short answer: SIP REFER will only work if the call itself is running over SIP. If your agent is handling calls via the native Twilio phone-number integration (WebSocket/TwiML), SIP REFER is not available—you can only use the Conference transfer type there. To use SIP REFER with a Twilio number, you must route the number to ElevenLabs via a SIP trunk (e.g., Twilio Elastic SIP Trunking) and ensure the trunk allows REFER. (elevenlabs.io)

What to check if you switch:

  • Connect the Twilio number to ElevenLabs through SIP trunking (ElevenLabs supports SIP trunks and includes Twilio-specific header fallback). (elevenlabs.io)
  • In the Transfer to Human tool, set the transfer rule’s type to SIP REFER (only available on SIP calls) and, if needed, add custom SIP REFER headers. (elevenlabs.io)

If you stay on the Twilio native integration and keep using Conference transfers, verify Twilio regional routing; mismatched regions can cause transfer operations to fail. (elevenlabs.io)

Sources:
Transfer to human | ElevenLabs Documentation
SIP trunking | ElevenLabs Documentation
Transfer to human | ElevenLabs Documentation
Twilio regional routing | ElevenLabs Documentation

cursive gull
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but SIP REFER worked?!!

fleet forgeBOT
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AI-generated Answer for p_hinn

Yes—SIP REFER is supported and will work when:

  • the call is running over SIP, and
  • your SIP trunk allows REFER. Only SIP REFER supports transferring to a SIP URI, and it performs a direct handoff to the destination. (elevenlabs.io)

If your tests with REFER succeeded, that’s expected. To use it reliably in production:

  • ensure you’re connected to ElevenLabs via SIP trunking (use the documented SIP URI format), and
  • provide E.164 phone numbers or valid SIP URIs for destinations; optionally add custom REFER headers if your carrier/system needs them. (elevenlabs.io)

Twilio can be connected via SIP to ElevenLabs Agents (as can Telnyx, PBXs, or self‑hosted SIP), so you can run calls over SIP and use SIP REFER in that setup. (elevenlabs.io)

If a transfer fails or hangs, check Call History for the failure reason to narrow it down. (help.elevenlabs.io)

Sources:
Transfer to human | ElevenLabs Documentation
SIP trunking | ElevenLabs Documentation
ElevenLabs — SIP Upgrades for ElevenLabs Agents
Where can I locate the reason for my call failing? – ElevenLabs

cursive gull
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And by the error description, that the caller remains inside Twilio with music without getting out of it, that would be prevented, right? The user would be handed off to the number that was transfered to?