Cannot answer calls on Android 11 / 12

>>> This item applies to Android 11 / 12 devices only <<< 

Starting in Android 11, they introduced new security features that require an extra step to Allow Display over other apps.  

When Allow Display over other apps is disabled [default setting], you cannot answer incoming calls when the overlay is presented.

Based on your phone manufacturer, the allow option may be located in a different location.
See below, some common manufacturers: