ons for any number 5. Caller ID Configuration In Retell, set the "Displayed Caller ID" option. For custom telephony via SIP trunk, the "User's Number" option for caller ID override is only supported for Retell-Twilio numbers, not custom SIP trunking. [(3) ]() [(1) ]() Next Steps: 1. Test if cold transfer works (if warm transfer is failing) 2. Check Twilio error logs for specific rejection reasons 3. Contact Twilio support with the error message to enable necessary transfer permissions [(1) ]()

​ To learn more, see the sources I considered: [[1] Warm transfer](https://discord.com/channels/1186424376300679209/1436106645632450671) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[4] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) ​

### Answer When using Twilio SIP trunking with Retell AI, transfer call failures typically stem from Twilio permissions rather than Retell configuration. [(1) ]() Common Issues & Solutions: 1. Verify SIP Trunk Settings Check your Twilio SIP trunk configuration, specifically the termination and origination settings. [(2) ]() [(1) ]() The issue is often on Twilio's side blocking the transfer to certain numbers. [(1) ]() 2. Transfer Type Matters - Cold transfer uses SIP REFER and may have different permission requirements than warm transfer [(3) ]() [(4) ]() - Warm transfer uses SIP DIAL and requires additional Twilio permissions [(1) ]() If cold transfer works but warm transfer doesn't, this indicates Twilio needs additional permissions enabled for warm transfer functionality. 3. Number Format Ensure your transfer destination number is in E.164 format (e.g., +12137771234) or use a SIP URI format like

sip:username@domain

. [(3) ]() [(4) ]() 4. Twilio Account Permissions For warm transfers specifically, you may need to: - Add the transfer destination number to your Twilio verified numbers list (especially for trial accounts) [(1) ]() - Contact Twilio support to enable warm transfer permissi

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

### Answer When transfers work for one number but fail for another, this points to number-specific restrictions rather than a general configuration issue. Check These Number-Specific Issues: 1. Number Format Validation Verify both numbers are in proper E.164 format (e.g., +12137771234) or valid SIP URI format (sip:username@domain) . [(1) ]() Inconsistent formatting between the working and failing numbers could cause selective failures. *2. Twilio Verified Caller IDs (Trial Accounts) * If you're on a Twilio trial account, you can only transfer to verified numbers. The working number may be verified while the failing one isn't. Check your Twilio console under "Phone Numbers" > "Verified Caller IDs" and add the failing number if needed. 3. Twilio Outbound Permissions Some numbers may be blocked by Twilio's geo-permissions or compliance rules. Check your Twilio account's: - Geographic permissions (some international numbers require explicit enabling) - Regulatory compliance settings - Any number-specific blocks or restrictions 4. Transfer Destination Number Type The failing number might be: - A toll-free number (requires special Twilio permissions) - An international number (check geo-permissions) - A VoIP/SIP number (may require different routing) - Behind an IVR system (may need Navigate IVR configuration) [(2) ]() [(1) ]() 5. Check Twilio Error Logs Review your Twilio console logs for the specific failed transfer. The error code will indicate whether it's: - Permission-related (403/451 errors) - Number validation issues (400 errors) - Routing problems (404/480 errors) Diagnostic Steps: 1. Test the failing number with a cold transfer i

nstead of warm transfer to isolate the issue [(1) ]() 2. Try transferring to the failing number from a regular Twilio call (outside Retell) to confirm Twilio can reach it 3. Check if the failing number has any special characters or extensions that need separate configuration [(2) ]() [(1) ]()

​ 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) ​

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