https://docs.vapi.ai/advanced/sip/sip-trunk "To configure the forwarding settings in Sonetel, you need to use your SIP username registered in VAPI. This username should be your phone number, followed by @ sip.vapi.ai (e.g., your-phone-number@sip.vapi.ai). For outbound calls, you'll need to set up an outbound authentication plan. This allows VAPI to make calls on your behalf using Sonetel. Similarly, for VAPI to accept incoming calls, authentication is required. You'll need to provide both a username and a password. The username should again be your phone number registered as the SIP username in VAPI, and the password can be anything as long as it’s not left blank or just whitespace. Integrating Sonetel with VAPI is feasible. For detailed instructions, I recommend referring to our SIP trunking documentation. This will give you a clearer understanding and help you proceed with the setup." -from a reddit post