sms_delivery_unavailable
The message could not be sent: the platform's vaulted messaging credential is missing or incomplete, or a carrier fault interrupted the send. An interrupted send is not retried here - the Messages endpoint takes no idempotency key, so a resend could deliver a second copy - so whether it went out is unknown and the caller starts over.
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/sms_delivery_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}