There's a YC company called Front that does more or less this: https://frontapp.com/

We use this at work for talking to customers and it works fine, but note that they use Twilio under the hood IIRC.

Also their SPA app is what you'd expect from JavaScript apps—buggy and prone to slowness once it reaches a certain scale.

WhatsApp would be huge. A WhatsApp PBX, auto-responders, etc. But it seems like FB would nix access pretty quickly so you'd constantly be playing whack-a-mole. They might even terminate the underlying account, making it too risky for businesses to even consider.