477 SIP Dial Failed Rate Excessively High

Support Help
6

A 65% dial failure rate with Telnyx through Retell requires immediate investigation. Based on recent community reports, this issue has affected multiple users.

Key Configuration Checks

First, verify your Telnyx elastic SIP trunking configuration:(1)

  1. FQDN Setup: Add Retell’s SIP server FQDN as sip.retellai.com with SRV as the DNS record type(1)

  2. Outbound Authentication: Select credentials as the authentication method and add the username and password(1)

  3. Required SIP Header: Telnyx requires the header X-Telnyx-Username: <username> to be included in outbound calls when using credentials as the authentication mechanism(1)

  4. Transport Method: Select TCP as the transport method (UDP temporarily not supported due to a connectivity issue)(1)

  5. Termination URI: When importing your number to Retell, supply Telnyx’s FQDN based on your SIP region choice (e.g., sip.telnyx.com)(1)

Common Root Causes

The telephony_provider_permission_denied disconnection reason represents cases where the SIP trunk authentication failed(2). Recent community cases have identified:(3)(4)

  • Unverified Origination Numbers: Telnyx error “403 Unverified origination number D51” indicates your originating phone numbers may not be verified in Telnyx(4)

  • Credential Mismatches: Username and password must match exactly between your Retell configuration and Telnyx trunk(3)

  • Missing Custom SIP Header: The X-Telnyx-Username header must be properly configured when importing the number or making outbound calls(5)(3)

Troubleshooting Steps

  1. Check Call History: Review the call detailed log for SIP error codes and error messages(2)

  2. Verify Credentials: Double-check your configuration and ensure you imported the correct username, password, and custom SIP headers(2)

  3. Contact Telnyx: Check with Telnyx support for errors on their side, specifically requesting they verify your origination numbers are properly verified(2)(4)

  4. Request PCAP Files: Ask Telnyx for PCAP files of failed calls with specific call IDs for deeper analysis(3)

If your configuration appears correct but calls still fail randomly, the intermittent nature suggests a provider-level authentication or routing problem rather than a pure configuration error(3).