Mine doesn't ring either. Just cuts off the music and displays the incoming call. This is with usb and bluetooth. My ringer is at full volume too. Perhaps a setting in the Android Auto app on the phone is the cause but I have not looked for it since I haven't missed any calls.
Connect phone. On right side, select phone options. Select your actual phone connection. Then there is a small button on right. Click this, it will show what your phone is enabled to do. Maybe this will show the issue.