Say you want to tell your mom once you safely reach school (via public transport). But you don't actually want to spend the money on a call or text (India did n

Say you want to tell your mom once you safely reach school (via public transport). But you don't actually want to spend the money on a call or text (India did not have "unlimited text", at least when I was growing up there). So your mom and you agree on a protocol that "one missed call means I reached safely". Now you just call her and disconnect immediately. Message received.