OK, a few observations.

1. I am not sure that the VBA MsgBox function actually calls the MessageBox API function, but that's not really critical.
2. a) API calls are usually far from simple.
b) When you get them wrong, Excel is liable to terminate with no warning at all- and that is if you are lucky.
3. There is no list anywhere of Excel functions that call APIs. I'd imagine most of them involve an API call at some point, since the API includes functions for memory management, Window drawing, Cursors etc. So I don't think the question makes sense as asked.
4. There is no Inputbox API call that I have ever seen.