Phone stopped ringing for ONE contact (only)

I finally found the SOLUTION! Disconnect from and FORGET your current WiFi network connection. REBOOT iphone so it boots without ANY WiFi connection. Rejoin your WiFi network as if it were new to you (you will need to enter the password, if not prompted then you didn't FORGET the network). Once connected with a fresh connection everything worked perfectly again. We had to do this on BOTH phones (as we both share the SAME home WiFi network). I know, it seems to make zero sense as we don't use WiFi for our calls but it has something to do with the "Private Address" feature (introduced in ios 14).

I found that if you simply disable (hit the little "i" net to your WiFi networks name while connected to see this option) this feature it will also correct the problem (that's how I traced the issue to the WiFi connection). This lead me to the better solution above (forget the network, reboot, rejoin as new). Much easier that a network reset (as you don't lose any connection information).