voice_session_unavailable
POST /v1/agents/{id}/voice-session: no browser voice-runtime endpoint is configured in this deployment (VOCAPABLE_VOICE_RUNTIME_BROWSER_URL), so there is no socket to connect to. The carrier-facing Media Streams endpoint is a different network path and never stands in for it - a session minted against it would hand a browser a credential for a host it cannot reach.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/voice_session_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}