### Answer The

telephony_provider_permission_denied

error indicates SIP trunk authentication failed. [(1) ]() A 20% failure rate with this error occurring intermittently (some calls work, some don't) is a pattern others have experienced. [(2) ]() Common causes: 1. SIP credentials mismatch — Double-check that your termination URI, auth username, and auth password exactly match what's configured with your telephony provider. [(1) ]() 2. Missing required SIP headers — If using Telnyx, you must include the

X-Telnyx-Username: <username>

header in outbound calls. [(3) ]() 3. Provider-side issues — The intermittent nature suggests the problem may originate from your telephony provider. One user with similar issues found Telnyx was returning error code D51 (unverified origination number) . [(4) ]() Troubleshooting steps: - Verify your SIP trunk credentials are correct [(1) ]() - If using Telnyx, ensure the

X-Telnyx-Username

header is properly configured when importing the number or making calls [(3) ]() - Check your call history detailed log for the SIP error code and error message [(1) ]() - Contact your telephony provider with specific failing call IDs to investigate on their end [(5) ]() - If configuration appears correct, delete and re-import the number [(6) ](

y) Which telephony provider are you using? That would help narrow down the specific configuration to check.

​ To learn more, see the sources I considered: [[1] Debug outbound connection issues](https://docs.retellai.com/reliability/debug-outbound-call) [[2] Large number of "Telephony_provider_permission_denied" errors](https://community.retellai.com/t/large-number-of-telephony-provider-permission-denied-errors/243) [[3] Telnyx](https://docs.retellai.com/deploy/telnyx) [[4] Telephony Permission Denied](https://discord.com/channels/1186424376300679209/1448901583399026728) [[5] " telephony permission denied" coming up again and again](https://discord.com/channels/1186424376300679209/1448951505431560276) [[6] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer Since you're using Twilio, here are the specific things to check for

telephony_provider_permission_denied

errors: 1. Termination SIP URI Issues - Check your Termination SIP URI for any spaces and remove them [(1) ]() - Use a localized termination URI near your region rather than the generic one 2. Credentials Verification - Verify your username and credentials are correct—note that the username is different from the friendly name shown in the credential list [(1) ]() [(2) ]() 3. Authentication Setup If using IP-based authentication instead of credentials, you need to whitelist Retell's IP addresses. Whitelist these IP ranges: [(3) ]() -

18.98.16.120/30

(all regions) -

143.223.88.0/21

(certain United States traffic) -

161.115.160.0/19

(certain United States traffic) *4. Geographic Permissions (for international calls) * - Search "geo" in Twilio to find "Voice Geographic Permissions" [(1) ]() - Choose "Elastic Sip Trunking" in the selector and select the countries you want to dial 5. Re-import the Number If configuration appears correct, try deleting and re-importing the number to Retell [(3) ]() Also check the logs in your Twilio dashboard for more details on why specific calls are failing, and consider opening a ticket with Twilio if the issue persists.

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Telephony provider permission denied](https://community.retellai.com/t/telephony-provider-permission-denied/542) [[3] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​