Outbound call with custom SIP trunk to Telnyx fails with 'telephony_provider_permission_denied'

Support Help
1

Outbound call with custom SIP trunk to Telnyx fails with ‘telephony_provider_permission_denied’. I’m using:

  • Termination URI: sip.telnyx.com

  • Username: 12345678

  • Password: [my password]

  • Custom SIP Header: X-Telnyx-Username: 12345678

  • Transport: TCP

The call never reaches Telnyx (no logs on Telnyx side). What could be wrong? I’ve tried everything.

Inbound Call Issue: number busy and registration failure with Messagenet trunk
2

From Monday, December 29, 2025, I can provide guidance on your Telnyx SIP trunk configuration issue.

The telephony_provider_permission_denied disconnection reason represents cases where the SIP trunk authentication failed(1). When using Telnyx with Retell AI, there are specific configuration requirements that must be met.

Configuration Requirements

For Telnyx elastic SIP trunking, you need to add Retell’s SIP server FQDN as sip.retellai.com with SRV as the DNS record type(2).

For outbound authentication, Telnyx requires the header X-Telnyx-Username: <username> to be included in outbound calls when using credentials as the authentication mechanism(2).

Likely Issues

Based on your configuration, here are potential problems:

  1. Incorrect Termination URI: You’re using sip.telnyx.com as your termination URI. According to the documentation, you need to find your specific FQDN based on your SIP region choice - it’s not just the generic sip.telnyx.com(2). Your FQDN should be region-specific (e.g., sip.telnyx.com for a specific region).

  2. Custom SIP Header Placement: The X-Telnyx-Username header needs to be configured when importing the number to Retell or when making outbound calls(2). One community member reported solving a similar issue by putting the SIP trunk username in the SIP header section when making the call(3).

  3. Credentials Configuration: When importing your number via the Import Phone Number API, you need to supply the username and password in the sip_trunk_auth_username and sip_trunk_auth_password fields(4).

Recommended Steps

  1. Verify your exact Telnyx FQDN based on your chosen SIP region from Telnyx’s documentation(2)
  2. Double-check that your SIP trunk credentials match exactly what’s configured in Telnyx(2)
  3. Ensure the X-Telnyx-Username header is properly configured as a custom SIP header(2)
  4. Contact Telnyx support to verify there are no issues on their end, as the error can originate from the provider(3)

If the configuration appears correct but calls still fail, check your Telnyx logs for any error messages and consider sharing a PCAP file with support for deeper analysis(3).