Creates a virtual fax line: a fax extension is allocated, the number in caller_id_number is pointed at it, inbound routing for that number is created or updated, and the users in users and emails_to_fax are given access. Each fax server uses one fax license, and the request is refused with 403 when none are left. The steps run one after another without a transaction, so a failure part-way leaves the earlier steps applied. Success is 200 rather than 201, and this requires an API key with the Admin role.