### Answer The error you're seeing ("Transfer call cannot be completed, probably don't have the permission to perform the transfer") when trying to show the original caller's number to the transfer recipient is likely related to caller ID override support with your telephony provider. [(1) ]() ## Key Points for Twilio + Caller ID Override When you configure the Transfer Call Tool to use "User's Number" as the displayed caller ID: - For cold transfer, Retell uses SIP REFER, and it's up to the telephony provider to support caller ID override for SIP REFER - For warm transfer, Retell uses SIP DIAL and sets

from

and

P-Asserted-Identity

headers to the user's number - If caller ID override is not supported, the transfer would fail Retell Twilio numbers should support showing the user's number on both warm and cold transfer. ## Twilio Configuration to Check 1. Verify SIP REFER and PSTN transfer settings - If you're using cold transfer with the transferee's number showing as caller ID, you need to configure your SIP trunking provider to allow SIP REFER and PSTN transfer [(2) ]() 2. Check Termination SIP URI - Ensure there's no space in it, and use a localized termination URI near your region [(3) ]() 3. Verify credentials - Make sure the username and credentials are correct (the username is different from the friendly name in Twilio) ## Recommended Troubleshooting - Try switching to warm transfer instead of cold transfer, as it has better support for caller ID override [(1) ]() - Temporarily test with "Retell Agent's number" as the caller ID to isolate whether the issue is specifically with caller ID

override - Check your Twilio logs to see what specific error is occurring on their side [(2) ]() If warm transfer works but cold transfer doesn't when using the user's number as caller ID, the issue is likely with how Twilio is handling the SIP REFER caller ID override for your specific trunk configuration.

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Twilio](https://docs.retellai.com/deploy/twilio) ​

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