### Answer To help troubleshoot your call transfer issues, here are the most common causes and solutions: Common Transfer Issues: 1. Custom Telephony/SIP Configuration If you're using custom telephony via SIP trunking, verify that your termination SIP URI is configured correctly without spaces and authentication is set up properly (IP whitelisting or credentials) [(1) ]() . The transfer destination number must be in the correct format [(2) ]() [(3) ]() . 2. Warm Transfer Permission Errors For warm transfers specifically, you may see errors like "Transfer call cannot be completed, probably don't have the permission to perform the transfer" [(4) ]() [(5) ]() . This typically indicates telephony provider limitations rather than Retell configuration issues [(4) ]() . 3. Caller ID Configuration If you're trying to show the user's number as caller ID during transfer, note that this feature is only supported for Retell-Twilio numbers . For custom telephony, you need to contact your provider about enabling this capability . 4. Transfer Number Format Ensure your transfer number is set as either: - A number in e.164 format - A SIP URI in the format

sip:username@domain

(e.g.

sip:user@retellai.com

) - A dynamic variable that gets substituted at runtime [(2) ]() [(3) ]() Debugging Steps: Check

the logs in your telephony provider to see where the call routing is failing [(1) ]() . If using Twilio with cold transfer and custom caller ID, note that Twilio does not honor custom SIP headers in the REFER request . Can you share more details about your setup (telephony provider, transfer type, error messages) so I can provide more specific guidance?

​ To learn more, see the sources I considered: [[1] Agent transfers the calls to itself](https://community.retellai.com/t/agent-transfers-the-calls-to-itself/699) [[2] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[3] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[4] Warm transfer](https://discord.com/channels/1186424376300679209/1436106645632450671) [[5] Discord message from @nicolas_46952 on 2025-10-29](https://discord.com/channels/1186424376300679209/1186462578310910112/1433217283928363059) ​

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