inbound_sms_callbacks_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.
The signed inbound-SMS callback plane has no credential vault or public base URL in this deployment, so the platform neither publishes a webhook URL nor tries to verify an incoming carrier request against a guessed origin.
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/inbound_sms_callbacks_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}