That error usually means Vapi can’t establish the media transport layer for the call (WebRTC/SIP). In most cases it’s tied to a config or connectivity issue like an invalid API key, a mismatch in your assistant or voice setup, or network restrictions blocking WebRTC (common on some servers, VPNs, or strict firewalls). I’d first double-check your API key and assistant configuration, then try running the call with a default voice to rule out provider issues. Also test from a different network to see if it’s environment-related. If you want, share your setup (without secrets) and I can help pinpoint exactly where it’s breaking. @jamillimaj