sms_delivery_failed
The carrier refused this specific message and retrying changes nothing - an unreachable landline, a handset that replied STOP, a number that cannot receive SMS. Nothing was delivered, and the caller must fix the destination rather than try again.
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_failed",
"title": "…",
"status": 502,
"detail": "…",
"instance": "/v1/…"
}