FaceTime: unable to answer incoming calls on macOS
Solution 1:
Try rebooting in safe mode as shown in this apple guide: https://support.apple.com/en-us/HT201262 and then restart your mac normally. This should clear the caches and fix the issue.
Source: https://medium.com/@ambroselittle/fix-call-failed-with-continuity-on-macos-sierra-e9e662ad6947
Solution 2:
I have the same problem as described above. I can make calls but can not receive any call from anyone. I have a new MacBook Pro so it rules out the possibility of the cache issue as described above. By the way, my mobile number is my AppleID.