answer_callback_unavailable
This code is raised on a non-customer surface (the /internal voice-runtime audience or telephony provider callbacks). It is excluded from the customer OpenAPI document and is registered here so an operator reading a log line has one place to look.
This replica cannot serve the voicemail-drop answer webhook: the webhook mints the call's own session token and points it at the Media Streams endpoint, and neither the session keyring nor the stream URL is wired on this process. A deployment fault, not the carrier's - the carrier may retry.
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/answer_callback_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}