Messages arrive as a conversation in the CRM. A first message from a number nobody
has spoken to creates a contact and a lead on the whatsapp source, with the
client's own words as the note — so an enquiry over WhatsApp is worked like any
other, and can be replied to from the CRM within Meta's 24-hour window.

Verified with x-hub-signature-256 against the app secret when one is configured.