### Answer I don't have explicit documentation on how Retell AI automatically handles SIP 486 Busy responses during cold transfers. [(1) ]() Here's what I can share: Cold Transfer Behavior - For cold transfer with a transferee number, Retell uses SIP REFER to transfer the call. [(2) ]() - Different telephony providers may or may not honor custom SIP headers in the REFER request. Agent Detection Timeout - Retell's warm transfer feature includes an "Agent Detection Timeout" that lets you configure custom timeouts to control how long the AI will wait for your agent to pick up before triggering fallback actions like sending a text, scheduling a follow-up, or rerouting the call. [(3) ]() However, this timeout feature is documented specifically for warm transfers, not cold transfers. Recommendation If you need more control over handling busy responses, consider: 1. Using warm transfer instead, which has built-in agent detection and timeout handling 2. Checking with your telephony provider's logs to understand the exact SIP signaling behavior [(4) ]() For deterministic call control and explicit SIP handling, Jambonz is mentioned as a SIP server option that can manipulate the SIP call flow. [(1) ]() Check out these potentially useful sources: [Custom Telephony Overview]() , [Transfer Call Function]() , [Transfer Call Node](

sfer-node) Would you like a general answer based on common SIP/VoIP practices?

​ To learn more, see the sources I considered: [[1] AI Voice Agent + SIP trunk: SIP BYE / end call handling](https://community.retellai.com/t/ai-voice-agent-sip-trunk-sip-bye-end-call-handling/764) [[2] Set & parse custom SIP headers](https://docs.retellai.com/build/telephony/sip-headers) [[3] Warm Transfer vs Cold Transfer - The AI Advantage in Call Centers | Retell AI](https://www.retellai.com/blog/effortless-handoffs-with-retell-ais-warm-transfer-feature) [[4] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​

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

### Answer Warm transfer uses SIP DIAL, not SIP REFER. [(1) ]() Here's the distinction between the two transfer types: - **Cold transfer**: Uses SIP REFER to transfer the call to a destination number. - **Warm transfer**: Uses SIP DIAL. For caller ID override on warm transfers, Retell sets the

from

and

P-Asserted-Identity

headers to the user's number. This is why warm transfer has additional capabilities like human detection, whisper messages, and three-way messages—the AI agent maintains control of the call during the transfer process rather than simply referring it away.

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) ​

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

Thanks for the feedback, glad I was helpful.