Large number of "Telephony_provider_permission_denied" errors

Support Help
1

Hi Retell team,

We are experiencing a large number of “Telephony_provider_permission_denied” errors. The issue occurs randomly for the same phone number—sometimes the call goes through, and other times we receive this error when dialing from the same number. We have not been able to identify any clear pattern or pinpoint when/where the issue is triggered. Overall, about 20–30% of our outbound calls fail with this error.

We are using a Telnyx FQDN SIP trunk. Could you please help us investigate and resolve this issue? It has a major impact on our business.

Thank you in advance.

477 SIP Dial Failed Rate Excessively High
2

The telephony_provider_permission_denied error indicates that outbound calls are not connecting because the SIP trunk credentials are not authenticated(1). This error typically means the telephony provider is rejecting the connection(1).

Based on your Telnyx FQDN SIP trunk setup, here are the key troubleshooting steps:

Verify Your Telnyx Configuration:

  1. Check Outbound Authentication Credentials(2)

    • Ensure the username and password you provided to Retell match exactly what’s configured in your Telnyx trunk(2)
    • Telnyx requires the header X-Telnyx-Username: <username> to be included in outbound calls when using credentials as the authentication mechanism(2)

  2. Confirm FQDN Setup(2)

    • Your FQDN should be sip.retellai.com(2)
    • DNS record type should be set to SRV(2)

  3. Verify Transport Method(2)

    • Telnyx documentation recommends using TCP as the transport method(2)
    • UDP is temporarily not supported due to connectivity issues(2)

  4. Check Outbound Voice Profile(2)

    • Ensure you’ve created a new outbound voice profile in Telnyx and selected it in the outbound settings(2)

Additional Considerations:

The intermittent nature of your issue (20-30% failure rate with the same numbers) suggests this may be a provider-level authentication or routing problem rather than a configuration error(3). Community reports indicate similar patterns where calls would fail randomly, then succeed on retry(3).

I recommend:

  • Double-checking all SIP credentials match between Retell and Telnyx
  • Verifying the custom SIP header (X-Telnyx-Username) is properly configured(2)
  • Contacting Telnyx support with specific call IDs and requesting PCAP files for failed calls(4)
  • Checking if your origination numbers are properly verified in Telnyx (error code D51 or D64 can indicate unverified numbers)(5)

If issues persist after verification, please reach out through the official support channels with specific call IDs for further investigation.

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