### Answer SIP 478 "Unresolvable destination" indicates the telephony provider cannot route the call to the destination number—typically meaning the number format is invalid or the carrier cannot resolve the destination. [(1) ]() ## Things to Check 1. Number Format - Ensure the transfer destination is in E.164 format (e.g.,

+12137771234

) or a valid SIP URI in the format

sip:username@domain

. [(2) ]() - If your transfer destination is not in E.164 format, you can choose to keep the input as raw format—but this only applies when using custom telephony. 2. Telephony Provider Configuration - If using custom telephony, double-check your configuration and make sure you imported the correct information. [(1) ]() - Check your termination settings in your SIP trunking provider and ensure you provided the correct termination URL to Retell. [(3) ]() - Review the logs in your telephony provider to see what error occurred on their side, and consider opening a ticket with them. [(1) ]() , [(3) ]() 3. Retell Settings - Verify the transfer number is correctly set in your Transfer Call Tool configuration. [(2) ]() - If the configuration appears incorrect, delete the imported number and re-import it. [(1) ]() 4. Destination Validity - The

invalid_destination

disconnection reason represents cases where the destination phone number is invalid. - Confirm the destination number is active and can receive calls

. Since the SIP 478 error points to a routing/resolution issue on the telephony provider side, I recommend checking with your carrier to investigate why they cannot resolve the destination.

​ To learn more, see the sources I considered: [[1] Debug outbound connection issues](https://docs.retellai.com/reliability/debug-outbound-call) [[2] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[3] 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.