Stops an outbound fax that has not finished sending. Only faxes in queued, media.processed or sending can be cancelled; anything else comes back as 422. On success the fax is marked cancelled with result code 98 and the reason Cancelled by user, an access entry is recorded, and connected apps on your account are notified. A fax that has already vanished upstream is treated as cancelled too, and the local record is still updated.