Great content just going to watch it through wishboneinvesors@gmail.com

This is huge — appreciate you dropping a full 1+ hour build-out from scratch 👏 I’m locked in on voice agents right now (Twilio + booking + follow-ups), so I’m going through this properly and taking notes. I’ll post my key takeaways + questions once I’m done.

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Yeah this error is almost always a trunk-side permission / auth mismatch, not Retell “breaking.” Since Twilio → Retell inbound works, your SIP trunk is reachable. The failure is on the Retell → Twilio outbound leg. Here’s the quick checklist that fixes “telephony provider permission denied” 9 times out of 10: 1. Twilio SIP Trunk auth / IP allowlistRetell outbound INVITEs will be rejected unless your trunk allows them. Retell docs say you must either: - use a Credentials List (username/password) on the trunk and enter the same creds when importing the number into Retell, or - whitelist Retell’s IP ranges (Retell doesn’t have one static IP, so you need to allow their published range).  If you set neither (or creds don’t match), Twilio will deny the INVITE → exactly your error. 2. Origination settings / E.164 formattingOn Elastic SIP Trunking, Twilio expects the INVITE Request-URI to reference your trunk’s Termination URI and the number in E.164. If Retell is dialing without the correct trunk termination URI or number format, Twilio can drop/deny it.  Make sure the number you imported into Retell is the same one assigned to the trunk and is in +1/+44… format. 3. Secure trunking / TLS mismatchIf you flipped Secure Trunking = ON in Twilio, any non-encrypted SIP from a peer will be rejected. That can surface as permission denied. Try turning it off to test, or ensure Retell is set to the matching secure mode. 4. Geo permissions / trial limitsTwilio will also deny outbound if the destination country isn’t enabled on your project (“Geo Permissions”). Even with a verified trial number, outbound to some regions is blocked until enabled. 5. Check Retell call logsRetell has a specific outbound debug page — the detailed log will show the SIP code Twilio is returning (403/407/etc.). That tells you instantly whether it’s ACL/creds/geo. TL;DR fix: Go to your Twilio Elastic SIP Trunk → add a Credentials List (or IP allowlist per Retell docs) → re-import/confirm the same creds in Retell → test with Secure Trunking OFF and E.164 numbers.