I don't see what's confusing about the app, both on macOS an iOS. You get a preview of your face, the last contacts you called and a green "New call" button.

To me this is simple to the point of being excessively simple, especially with the "missed calls" being in a 16x16 click target dropdown.

It looks like they optimized for "calls from grandma" mode.

You don't see what's confusing, and then you proceed to mention something confusing. If a basic-skills user can't find "missed calls", that's confusing!