### Answer TCP is recommended over UDP for SIP trunking in Retell AI for a few reasons: Documented Recommendation The Retell documentation lists TCP as the recommended transport method for SIP trunking. [(1) ]() Provider-Specific Issues For Telnyx specifically, the documentation notes to select TCP as the transport method, with UDP "temporarily not supported due to a connectivity issue." [(2) ]() Troubleshooting Experience Community discussions indicate that switching from UDP to TCP has resolved outbound SIP issues for some users experiencing connection problems. [(3) ]() General Benefits of TCP TCP provides reliable, connection-oriented communication which ensures SIP signaling messages are delivered in order and without loss. This is particularly important for call setup and teardown where missing or out-of-order messages can cause call failures. To use TCP transport, append the transport method to the SIP server URL:

sip:sip.retellai.com;transport=tcp

[(1) ]()

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Telnyx](https://docs.retellai.com/deploy/telnyx) [[3] SIP Trunk Outbound Call - No SIP INVITE reaching my server](https://community.retellai.com/t/sip-trunk-outbound-call-no-sip-invite-reaching-my-server/760) ​

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