Got it that error confirms the issue is on the SIP connection or how VAPI is handling the outbound INVITE. I’ve fixed this exact problem before usually it’s due to how the trunk is registered or how the transport (UDP/TCP/TLS) is set up. If you’d like, I can take a closer look at your VAPI and Issabel configuration to identify the exact cause and make it stable again. Would you like me to handle the fix directly for you?