Hello diverter,
To get started, we advise checking the settings here, just to make sure you've reviewed all the needed configuration to answer calls on your Mac: Make and receive phone calls on Mac or iPad - Apple Support.
If everything is configured correctly and it should be working but just is not, we advise that you first turn off the "On your iPhone, go to Settings > Phone > Calls on Other Devices, then turn on Allow Calls on Other Devices." Then turn it back on. You may also need to do this on your Mac.
If after those steps, you're still having the issue, go ahead and restart your Mac (Shut down or restart your Mac) and make sure it's fully updated (Update macOS on Mac). Once those are done, please test this issue.
If the issue persists, continue by testing to see when this behavior occurs. First, put your Mac into safe mode and test. Booting into safe mode will disable any startup and login items from automatically opening, which can help us to isolate the source of the issue further. When you do this, you’ll see some unusual behavior from the display; this is to be expected. Please follow the steps in this article: How to use safe mode on your Mac.
Next, let's see if this behavior persists in a new user account on your Mac. Testing in a new user will determine if you have a system-wide issue or if it's user-specific. Please follow these steps to make a test account that can be deleted later: Change Users & Groups settings on Mac. Once the new user account has been set up, log out of your own account then into the new one to test the issue.
Let us know how this goes, please. Thanks and take care.
