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

The document must be reachable at attachment_url using HTTPS.

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.