Sends a fax using the fax channel that matches the from phone number.

Provide body (plain text, converted to a PDF at send time), attachments (HTTPS URLs to
PDF/TIFF or similar documents), or both. One of body or attachments is required. When both are
present, the body PDF is sent first, then each attachment in order.

Each attachments URL must use HTTPS. Whippy downloads your files, stores copies in your
organization storage, and sends the fax from those Whippy-hosted URLs (same model as SMS/MMS
attachments). Scheduled sends re-host attachments when the request is accepted; body-only faxes
are composed when the scheduled job runs.

If there is no contact for to, a new contact is created along with a fax conversation on the from channel.

Optional schedule_at queues the fax for a future time (ISO 8601). Optional conversation_status
can set the conversation to automated, open, or closed when sending.

Reading inbound and outbound fax: Use the same Conversation APIs as for SMS—list or show a conversation
and its messages to view fax history; there is no separate fax-only read API.