What is installed when using a SaaS solution?

Well there is initial setup at the very least. Hooking up whatever landline the client has with the SaaS solution.

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...

Also, whatever the SaaS system is, I bet you it's significantly less efficient than a receptionist using paper and desktop software.

Web systems are almost never built for productivity.

It is customised against each business model. Some are same, some are different.

The issue isn't universal API, but universal data models which is probably impossible.