Orders a phone number returned by a recent availability search and starts billing for it. This spends money and cannot be undone through this API — to stop paying for an ordered number, disconnect it. The number must still be in the results saved by POST /api/v1/numbers/available/local or /toll_free; one that has aged out of that storage is a 404, so search again and reorder. A 200 means the carrier accepted the order, not that the number is provisioned and taking calls. A key with no billing customer cannot order, and sending customerId does not work around that. Requires an API key with the Admin role.