Hello! I have the same problem with my Mac. I receive FaceTime calls on my iPhone but at the same time, I don't get FaceTime call notification on my MacBook Air M1. I tested this in any way possible - when it comes to receiving phone calls on Mac, everything works as it should - I can answer the phone call on my Mac without any trouble. The problems starts when somebody tries to call me on FaceTime. Then I receive call only on my iPhone.
After some testing I realized that the issue happens with 2 of 3 contacts tested. I can't find the reason why it happens. Maybe it's because of iOS version? The contact that works fine has 15.0 iOS, but the problematic one has 15.0.2.