### Answer The sources don't have specific documentation on error 32204 (invalid caller id) for SIP transfers on Twilio. What is documented is that showing the caller's original number on a transfer requires caller ID override support from your telephony provider, and this varies by transfer type: Retell Twilio numbers support caller ID override on both warm and cold transfer, while Retell Telnyx numbers only support it via cold transfer (SIP REFER) [(1) ]() . If override isn't supported for your configuration, the transfer fails . The fix documented is to switch the Displayed Caller ID setting to Retell Agent's Number instead of trying to pass through the original caller's number, or use a number/transfer type combination that supports override . I don't have direct documentation on the specific 32204 error code itself. Since you're getting a Twilio-side error, checking your termination settings in the SIP trunking provider and reviewing your provider's logs is also recommended for transfer-related issues [(2) ]() . Would you like me to give a more general answer based on common SIP/Twilio caller ID validation behavior, outside of what's directly documented here?

​ To learn more, see the sources I considered: [[1] Transfer call tool (single & multi-prompt agents)](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Connect Retell voice agents to custom telephony](https://docs.retellai.com/deploy/custom-telephony) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.