verified_number_delivery_unavailable
No messaging provider is composed in this deployment to deliver the one-time code to the number, so registration refuses rather than minting a code nobody can receive. Delivery rides the platform Twilio account (ADR-0005) and needs both the vaulted platform credential and a sending number. Landlines still have no voice readout (03-domain-model.md §Code delivery); an SMS-only deployment simply never confirms one.
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/verified_number_delivery_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}