Disclosure: I work on Cal ID. We shipped WhatsApp booking a while back and this is the list of things that broke along the way.
"Let people book appointments without leaving WhatsApp" sounds like a small feature. It is not.
WhatsApp was not designed as a booking surface, and a lot of what you assume works either doesn't, or works in a way that quietly produces wrong data. Most of these we found the expensive way, in production, after shipping.
Here's the list, roughly in order of how much time each one cost us.
WhatsApp Flows do not render on Desktop, and the API tells you everything is fine
