I'm sure there's plenty of RF energy to make a passive phone case that lights up when your phone is transmitting, which would detect this case if you were looki

I'm sure there's plenty of RF energy to make a passive phone case that lights up when your phone is transmitting, which would detect this case if you were looking at your phone. Maybe having it emit a tone would be better. It would be fun to see if you could get a modern phone to shut up for even five seconds when powered on.