That doesn't sound right. I think you'd pay something like $99 per month for a SaaS product that manages bookings and provides an API. That's how much the average receptionist earns in one day ($12.25 per hour.)
* Most businesses already have a receptionist.
* Most receptionists do not spend measurable fractions of their day answering phonecalls asking when the business is open.
* Taking bookings is really also not the majority of their day.
* Receptionists are capable of a bunch of things that your SaaS booking program is not. Like ordering catering and picking up office supplies.
* A SaaS booking program that is looked over by a human doesn't have to have AI-systems, because they just have I-systems. A human receptionist.
* The inevitable job-post catchall "Other duties as required."
* I had a receptionist bring me a beer once while I was waiting and I'm pretty sure none of your SaaS solutions will do that.
Then whenever a significant change is required, you need to call back your "expert". New location? xk$, etc.
But I think the biggest concern is that suddently the owner does not understand how his reversation system works. He used to be able to call Joe and know what's going on...
Web systems are almost never built for productivity.
The issue isn't universal API, but universal data models which is probably impossible.