### Answer --- For inbound calls with custom telephony (Twilio UK number) , here are the key troubleshooting steps: ## Check Your Origination Settings For inbound calls to work, you need to configure your SIP trunk's origination settings correctly. [(1) ]() The Retell SIP server URI is:

sip:sip.retellai.com

For transport methods, you can use: - TCP (Recommended) :

sip:sip.retellai.com;transport=tcp

- UDP:

sip:sip.retellai.com;transport=udp

- TLS:

sip:sip.retellai.com;transport=tls

## IP Whitelisting You may need to whitelist Retell's IP block in your Twilio configuration: -

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) ## Verify Your Import Configuration When importing your Twilio number, ensure you've provided the correct

termination_uri

(for outbound) and that an

inbound_agent_id

is set. If

inbound_agent_id

is null, the number will not accept inbound calls. [(2) ]() ## Common Troubleshooting Steps If inbound calls are not connecting: [(1) ]() 1. Check your origination setting in your SIP trunking provider (Twilio) 2. Check the logs in your telephony provider 3. Consider opening a ticket with Twilio if the issue persists Could you share what specific error or behavior you're seeing (e.g., disconnection reason, error messages in Twilio logs) ? This will help narrow down the issue.

​ 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) ​

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

### Answer Given the busy signal behavior you're experiencing, there are a few additional things to check based on similar reported issues: ## Try TLS Transport A community member reported a similar busy signal issue with inbound calls on a Twilio Elastic SIP Trunk. [(1) ]() The suggestion was to use TLS transport specifically:

sip:sip.retellai.com;transport=tls

You mentioned trying UDP and TCP, but TLS may behave differently with Twilio's infrastructure. ## Verify Number Import Details When importing your number to Retell, ensure: [(2) ]() - The termination SIP URI is correct (use a localized termination URI near your region) - If you set up auth via credentials, the username and password are supplied correctly - Note: The username is different from the "friendly name" shown in Twilio's credential list ## Check Twilio Logs The documentation recommends checking the logs in your telephony provider when inbound calls aren't connecting. [(3) ]() Twilio's call logs should show the SIP response codes, which can help identify whether the issue is on the Twilio side or the Retell side. ## Consider Reaching Out to Support Since you've exhausted the standard troubleshooting steps and the issue persists, this may require direct investigation. You can: - Visit [Contact Sales]() - Check [Retell AI GitHub]() Would you like me to provide a general answer on other potential causes, or have you checked Twilio's SIP logs for specific error codes?

​ To learn more, see the sources I considered: [[1] Elastic SIP Trunk Busy Signal](https://discord.com/channels/1186424376300679209/1415758706133696562) [[2] Twilio](https://docs.retellai.com/deploy/twilio) [[3] 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.