Verify the signed Vonage Verify callback, durably accept it, then return `200` or `204`. Vonage documents retries after a non-2xx response. Run slow processing after durable acceptance so acknowledging a callback does not lose work.

Make repeated delivery of the same notification safe. Do not discard every later callback with the same `request_id`: one verification can produce event and summary callbacks carrying different state. Track the notification or state transition as well as the request, and make each side effect idempotent. Test both a repeated callback and distinct progress/completion callbacks for one request.