Looking at your diagram, the issue is clear: Vapi is sending a "503 Service Unavailable" response to voip.ms, which is why the call isn't connecting. This typically means either Vapi's service is temporarily down, your Vapi configuration isn't properly set up to receive calls, or there's an authentication/endpoint issue. Quick question: Have you verified that your Vapi phone number is properly configured and active? Also, are you using Vapi's provided SIP trunk settings or trying to route through your own server?