### Answer In Retell AI's call transfer context, the difference relates to how the two transfer types work: Cold Transfer (uses SIP REFER) : - The call is transferred to a destination number and that's it. [(1) ]() [(2) ]() - For caller ID override showing the user's number, it's using SIP REFER, and it's up to the telephony provider to support caller ID override for SIP REFER. - Retell Telnyx numbers only support showing the user's number when using SIP REFER via cold transfer. Warm Transfer (uses SIP DIAL/INVITE) : - After the call is transferred to the destination number, the AI agent can attempt to detect if the other side is human, leave private messages that are not heard by user, do a three-way introduce, etc. - For warm transfer, it's using SIP DIAL, and Retell sets the

from

and

P-Asserted-Identity

headers to the user's number. - Retell Twilio numbers support showing user's number on both warm and cold transfer. Key practical difference for caller ID: - Warm transfer (SIP DIAL) gives Retell more control over headers for caller ID passthrough - Cold transfer (SIP REFER) depends on whether your telephony provider honors custom SIP headers in the REFER request [(3) ]()

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[3] Help on Twilio DID questions](https://retell.discourse.group/t/help-on-twilio-did-questions/24) ​

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

### Answer For SIP REFER (cold transfer) , which caller ID is displayed depends on your configuration and telephony provider support: Caller ID Options: You can configure which caller ID shows up to the transfer destination: [(1) ]() [(2) ]() - **Retell Agent's number**: The transfer destination will see the Retell agent's number - **User's Number**: The transfer destination will see the number of the user Important Limitation for SIP REFER: For cold transfer using SIP REFER, it's up to the telephony provider to support caller ID override. Provider-Specific Behavior: - Retell Twilio numbers support showing user's number on both warm and cold transfer - Retell Telnyx numbers only support showing the user's number when using SIP REFER via cold transfer - If caller ID override is not supported by your provider, the transfer would fail Custom Telephony Consideration: Twilio does not honor custom SIP headers in the REFER request for cold transfers with transferee caller ID. [(3) ]() So the short answer: SIP REFER can display the user's number if you select that option AND your telephony provider supports caller ID override for SIP REFER. Otherwise it will show the Retell agent's number or the transfer may fail.

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[3] Help on Twilio DID questions](https://retell.discourse.group/t/help-on-twilio-did-questions/24) ​

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