I have been using Twilio for long time and recently started exploring Telnyx. It is much better to show the SIP call flow, logs and debugging.
Just sharing the two days worth of troubleshooting with Telnyx to save you time and effort if you encounter these two issues in Retell AI with a Telnyx phone number
Inbound calls going busy -- Change the phone number format to E.164 or +E.164 under SIP trunk inbound routing settings.
2. Outbound calls not connecting - just use "sip.telnyx.com" instead of FQDN which include the SIP trunk name in the URL on the retell side under SIP URL field (when you import a phone number)