voicemail_callback_unavailable
The callback_e164 a voicemail message would name is not an active, voice-capable number assigned to this tenant. The opt-out number a message reads aloud has to be one a callee can actually reach, so it is validated when the agent is saved rather than when the first message is left.
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/voicemail_callback_unavailable",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}