Able to answer—but not make—iPhone calls on Mac
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 &amp; 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