### Answer Retell AI uses *SIP (Session Initiation Protocol) * for telephony connections, not WebSocket URLs starting with "wss://". [(1) ]() For connecting your telephony provider to Retell AI via SIP trunking, you need to use Retell's SIP server address, which is

sip.retellai.com

. When setting up Elastic SIP Trunking with your provider: 1. Add Retell's SIP server FQDN:

sip.retellai.com

[(2) ]() 2. Configure authentication using username and password credentials 3. Set the transport method (TCP is recommended) After configuring your SIP trunk, you'll need to import your phone number into Retell using the termination URI from your telephony provider. [(3) ]() Note: Retell's SIP server does not have a static IP address as it's distributed globally, so you'll need to use credential-based authentication (username/password) rather than IP whitelisting. [(4) ]() If your provider is specifically asking for a WebSocket URL (wss://) , they may be confusing SIP trunking with Retell's Custom LLM WebSocket integration, which is a different feature for connecting custom language models, not for telephony connections. For detailed setup instructions specific to your telephony provider, check the [Custom Telephony Overview]() documentation. [(1) ]()

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] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Telnyx](https://docs.retellai.com/deploy/telnyx) [[3] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) [[4] SIP Trunking](https://discord.com/channels/1186424376300679209/1389640341476610219) ​