People turn off their cell phones when they really don't want to be disturbed, or sometimes in anticipation of the "turn off your devices" rule of air travel, o

People turn off their cell phones when they really don't want to be disturbed, or sometimes in anticipation of the "turn off your devices" rule of air travel, or it's just a dead battery. What's an AI to do when wanting to talk to someone who's phone is off? ring every cell phone he walks by?