### Answer To help troubleshoot your transfer call issue, here are the key things to verify: ## Basic Requirements - Transfer call only works during phone calls (not web calls) and is available for Retell numbers and imported numbers. [(1) ]() [(2) ]() - The transfer number must be in E.164 format (e.g.,

+14155551234

) or a SIP URI in the format

sip:username@domain

. ## Common Issues to Check 1. Transfer Type Configuration - **Cold transfer**: Call is transferred directly to the destination. - **Warm transfer**: Includes human detection, whisper messages, and other advanced features. 2. Caller ID Settings If you're using "User's Number" as the displayed caller ID: - For cold transfers, this uses SIP REFER and depends on your telephony provider supporting caller ID override. - If caller ID override is not supported, the transfer will fail. - Retell Twilio numbers support showing user's number on both warm and cold transfer. 3. Custom Telephony / SIP Trunking If using custom telephony, verify your Termination URI is configured correctly—it should be the URI from your provider, not Retell's URI. [(3) ]() 4. Warm Transfer with Custom Telephony Warm transfer with "User's Number" caller ID is fully supported only for Retell-purchased numbers. For custom telephony, you may need to contact your provider for additional permissions. [(4) ]() ## Recommended Troubleshooting Steps 1. Try switching caller ID to "Retell Agent's number" to test if transfers work [(5) ]() 2. If using warm transfer, try cold transfe

r first to isolate the issue [(5) ]() [(4) ]() 3. Check your detailed call logs for specific SIP error codes [(3) ]() Could you share more details about your setup—are you using Retell numbers or custom telephony? And are you attempting warm or cold transfers?

​ 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] Not being able to transfer a call](https://community.retellai.com/t/not-being-able-to-transfer-a-call/225) [[4] Warm transfer](https://discord.com/channels/1186424376300679209/1436106645632450671) [[5] Call transfer failures](https://community.retellai.com/t/call-transfer-failures/600) ​

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