#i'm trying to transfer to a human but i get the following message in the analytics transcription: "

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stray larkBOT
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AI-generated Answer for gogetter7

Likely causes and fixes for “Agent failed to transfer the call to Unknown number”:

  • The transfer target isn’t configured on the agent. The transfer_to_number tool requires you to add explicit transfer rules and the LLM’s transfer_number must match one of those configured numbers. Add the “Transfer to number” system tool and define the destination(s); ensure the number is in E.164 format (e.g., +12125551234) or a valid SIP URI. (elevenlabs.io)

  • You’re using Twilio “register call” instead of the native Twilio import. Transfers are not supported with the register-call approach, so any transfer attempt will fail. Use the native Twilio integration (import your number and link it to the agent) if you need transfers. (elevenlabs.io)

  • Twilio regional routing mismatch. If your imported Twilio number is routed to a region (e.g., ie1, au1) but the ElevenLabs number config doesn’t specify the same region, operations like transfers will fail. Set the correct Twilio region in ElevenLabs and retest. (elevenlabs.io)

  • SIP setup constraints. If you’re transferring via SIP, only SIP REFER supports SIP URIs and your SIP trunk must allow REFER; otherwise use a phone-number transfer. (elevenlabs.io)

Where to confirm the exact failure reason:

  • Check Calls History/Analytics for the red failure message and error details to pinpoint the cause. (help.elevenlabs.io)

If you share which telephony path you’re using (native Twilio import, Twilio register-call, or SIP trunk) and the exact transfer rule you configured, I can point to the precise setting to fix.

Sources:
Transfer to number | ElevenLabs Documentation
Register Twilio calls | ElevenLabs Documentation
Twilio regional routing | ElevenLabs Documentation
Transfer to number | ElevenLabs Documentation
Where can I locate the reason for my call failing? – ElevenLabs

cerulean basin
stray larkBOT
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AI-generated Answer for gogetter7

Thanks—that context helps. With Vonage over a custom SIP trunk, that “Agent failed to transfer the call to Unknown number” message is almost always one of the following configuration issues:

  • The transfer target isn’t defined in your agent’s Transfer to number tool, or the LLM tried a number that doesn’t exactly match one of your configured rules. The tool requires transfer_number to match a preconfigured destination. Define explicit rules and ensure the number format is correct. For phone numbers use E.164; for SIP use a full SIP URI. (elevenlabs.io)
  • If you selected SIP REFER as the transfer type, your SIP trunk must permit REFER. REFER transfers only work when the conversation is running over SIP and the trunk allows REFER; warm-transfer messages aren’t supported over SIP. If REFER isn’t enabled by your carrier, use the default conference transfer to a phone number instead. (elevenlabs.io)
  • Number formatting mismatch. Keep the leading “+” convention consistent and use E.164 in both your transfer rule and the associated phone configuration; mismatches can prevent routing. (elevenlabs.io)
  • General SIP-trunk setup pitfalls: ensure you’re using TCP/TLS (UDP isn’t available), and that outbound calls from ElevenLabs are pointed at your SIP trunk hostname. (elevenlabs.io)

Notes specific to Vonage

  • Vonage is a supported SIP provider for ElevenLabs SIP trunking, so the above applies. (elevenlabs.io)
  • If you’re integrating via Vonage’s Voice API instead of pure SIP, ElevenLabs also provides a WebSocket connector pattern; transfer behavior then depends on your app flow rather than SIP REFER. (elevenlabs.io)

Where to confirm the root cause

  • Check the call’s entry in Call History/Analytics for the failure reason to verify whether the destination was missing/misformatted or the transfer type was unsupported. (help.elevenlabs.io)

Action checklist

  1. In your agent, open Tools → Transfer to number and add an explicit rule for the exact destination you want (E.164 or SIP URI). (elevenlabs.io)
cerulean basin
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how come that when i do an outbound test call to a specific e164 number, the call initiated successfully, but asking the agent to transfer to an e164 number, it fails to do so .