#Hi I have a agent call initiated through a sip trunk and want to enable transfer_to_number. I have

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tidal lilyBOT
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AI-generated Answer for tututim_1

Here are the checks that resolve most transfer_to_number failures with SIP trunks:

  1. Verify the destination is configured exactly in your agent
  • In Tools > Transfer to Human, add a rule for the number and ensure:
    • Number type = Phone (not SIP URI) for Conference transfers.
    • Phone number is in E.164 format (e.g., +12125551234).
    • The LLM’s tool call must pass transfer_number that exactly matches one of the configured numbers; mismatches will fail. (elevenlabs.io)
  1. Pick a transfer type your trunk supports
  • Conference (default): calls the destination and bridges via a conference. Works with phone numbers.
  • SIP REFER: transfers the SIP leg directly; requires the call to be on SIP and your SIP trunk to allow REFER. Only SIP REFER supports transferring to a SIP URI. If REFER is disabled at the provider, REFER transfers will fail. (elevenlabs.io)
  1. If using Conference transfers over SIP, confirm outbound trunk settings
  • In your SIP trunk config, set Outbound “Address” to your provider’s hostname/IP (do not enter a full SIP URI).
  • Outbound calling may be limited by your SIP provider; verify they allow calls to the destination and from your caller ID. (elevenlabs.io)
  1. Validate the tool call parameters before you place real calls
  • Use Agent Testing to create a Tool Call Test for transfer_to_number and enforce an Exact Match or E.164 regex for transfer_number, so you can see if the LLM is sending the number in the required format. (elevenlabs.io)
  1. Check the call failure reason shown in Call History
  • The Call History page surfaces red error messages explaining why the call/transfer failed; review that exact reason on the failed attempt. (help.elevenlabs.io)
  1. Allowed destinations and formatting
  • You can transfer to external numbers reachable via your SIP trunk or Twilio. Ensure phone numbers are E.164 and SIP URIs are only used with SIP REFER. (elevenlabs.io)

If you still see “failed to transfer … Unknown number,” it almost always indicates the transfer_number sent by the agent

little solar
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The LLM’s tool call must pass transfer_number that exactly matches one of the configured numbers; mismatches will fail. (elevenlabs.io)
what does this mean