contact_opted_out

That number replied STOP, so it is never notified again and cannot be re-added.

What to do

The opt-out is one-way on this surface, deliberately — otherwise delete-and-recreate would undo it. The frozen record is what makes the refusal possible.

The envelope

{
  "error": {
    "type": "unprocessable_error",
    "code": "contact_opted_out",
    "message": "…",
    "hint": "The opt-out is one-way on this surface, deliberately — otherwise delete-and-recreate would undo it. The frozen record is what makes the refusal possible.",
    "retriable": false,
    "doc_url": "https://docs.facturia.cl/errors/contact_opted_out",
    "request_id": "req_01K2R7Q9M4T2VB"
  }
}

Program against code, never against message — the prose can be rewritten, the code cannot (§5.8).

hint and retriable arrive in the response with these exact values: they are a function of code and come from the same dictionary GET /v1/error_codes serves, so the envelope and the dictionary cannot disagree. Both are English on the wire by design — one registry, one language. More in The error envelope.

See also

The machine-readable copy of this page lives at GET /v1/error_codes, unauthenticated.