Submit your details and documents (multipart/form-data). Parts: `country_code`, `number_type`, `subject_type`, `provider` (optional), `fields` (JSON object), `address` (JSON object, when required), `documents` (JSON object: slot name to `{"option": ..., "fields": {...}}`), and one file part per slot named `file.<slot>`. Get the slot and field names from `GET /verifications/requirements`. The carrier checks the details right away. Problems come back as 422 with the field named, and nothing is kept. When it passes, the verification is submitted to the carrier for its review; read it back to follow the state.