You attempted a SIP REFER transfer while the call was running over the native Twilio integration. SIP REFER transfers are only available when the call itself is using SIP (via a SIP trunk) and the trunk allows REFER; the Twilio-native path supports conference (default) and blind transfers instead. Hence “Unsupported transfer type for Twilio: sip_refer.” (elevenlabs.io)
How to resolve
If you are staying on native Twilio numbers:
Change the transfer rule’s transfer_type to conference (default) or blind (blind is only available when the Twilio number is imported via the native Twilio integration and is configured via the JSON editor). (elevenlabs.io)
If you need SIP REFER (e.g., to transfer to a SIP URI):
Move the call to a SIP trunk integration (or Twilio Elastic SIP Trunking) so the conversation runs over SIP, and ensure your SIP trunk permits REFER; then use transfer_type: sip_refer. (elevenlabs.io)
About “Unknown number”
The tool call must provide transfer_number that exactly matches one of the preconfigured transfer rules, and phone numbers must be in E.164 format; otherwise the transfer will fail. Verify the destination is defined in your rules and formatted correctly. (elevenlabs.io)
Note if you are using the Twilio “register call” pattern instead of importing the number natively: call transfers are not supported in that mode. (elevenlabs.io)
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