My father worked for the telephone company and he would occasionally demonstrate how phones worked. One lesson I remember is how you could use sound to dial phone numbers. I was pretty young, so this seemed like magic to me, but my sister who was older, wiser and more inquisitive asked if it was possible for the phone to hear a noise and dial a number by mistake. He explained how this was not easy because when you pushed the button you were actually “hearing” two tones being played at the same time.
I had always remember him saying this, but never really delved deeper into what he meant until yesterday when I was reading an old copy of Popular Mechanics and found this explanation that sums up the lesson I think he was trying to teach us.