Able to answer—but not make—iPhone calls on Mac
Solution 1:
Try completely logging out of your Apple ID/Account, everywhere (every setting/option where you can do so), on both of your devices.
For example, on iOS, you should try signing out from iCloud, iMessage and FaceTime:
- Settings → iClound → Sign Out
- Settings → Messages → Send & Receive → Apple ID: ... → Sign Out
- Settings → FaceTime → Apple ID: ... → Sign Out
Then try logging back into your Apple ID/Account on them and setting it up again.
Hope this helps :)
-James