### Answer Based on the issue you're describing—no audio on SIP-connected landline numbers but mobile works—here are the steps to troubleshoot: 1. Check the call history and detailed logs Go to your Retell Dashboard and check the

disconnection_reason

field and detailed logs for the affected calls. [(1) ]() Look for error codes like: - `error_no_audio_received`: Retell has not received audio from the telephony side after connection - SIP error codes that may indicate routing or configuration issues 2. Verify your SIP trunking configuration If using custom telephony (elastic SIP trunking) : [(2) ]() - Double-check your configuration and ensure you imported the correct information - If the configuration is incorrect, delete the imported number and re-import [(1) ]() - Check with your telephony provider to see if there's an error on their side 3. Verify termination URI and authentication When importing a phone number, ensure the

termination_uri

is correct for your SIP trunk provider. [(3) ]() Also verify any

sip_trunk_auth_username

and

sip_trunk_auth_password

credentials are accurate. 4. Check transport protocol The default transport is "TCP", but you can also use "TLS" or "UDP". [(4) ]() If your provider requires a specific transport, update your phone number configuration accordingly. Since the mobile number works but landlines don't, this suggests the issue may be specific to how your SIP provider routes calls to landline numbers. I'd recommend checking the logs on your telephony provider's side for those specific calls.

​ To learn more, see the sources I considered: [[1] Debug outbound connection issues](https://docs.retellai.com/reliability/debug-outbound-call) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) [[4] Update Phone Number](https://docs.retellai.com/api-references/update-phone-number) ​

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

@AI Evy The land line number was working fine but suddenly broke

### Answer Since the landline number was working and suddenly stopped, here are the key things to check: 1. Check your SIP trunk provider's side Retell will not throw errors if the SIP configuration becomes incorrect—it won't know until a call is made. [(1) ]() Check the logs in your telephony provider and consider opening a ticket with them. 2. Verify nothing changed in your SIP trunk settings For outbound calls not connecting, check your termination settings in your SIP trunking provider and ensure you provided the correct termination URL to Retell. 3. Re-import the number if needed If you need to change the configuration of an imported number, you currently need to delete and re-import it. When re-importing, ensure the

termination_uri

and authentication credentials (

sip_trunk_auth_username

,

sip_trunk_auth_password

) are correct. [(2) ]() 4. Check transport protocol Valid transport values are "TLS", "TCP", and "UDP" with "TCP" as default. [(3) ]() If your provider changed requirements, you may need to update this. Since this worked previously and suddenly broke, the issue is likely on your telephony provider's side (expired credentials, configuration changes, routing issues to landlines) . I'd recommend checking with them first.

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) [[3] Update Phone Number](https://docs.retellai.com/api-references/update-phone-number) ​

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