Back in the 60s you could dial your own number and of course you would get the beeping busy signal with quiet between the beeps. What people discovered was that if two or more people did that, their phones were actually connected together, and you could talk between the beeps and communicate that way.
Eventually, the phone company discovered what was happening, and they changed the busy signal to be a constant tone with louder beeps. That ended the party.
I did not know that! Wish I could go back and play with it.
I think there ws a secret number that would tell you the phone number you were calling from, and another one that would phone you back as a test.
I’ve wondered if there were any filters on pay phones to stop someone from playing back the coin chimes from a tape.
we used the phones as intercoms in the house.
I cannot remember what we dialed but it made the phones of that number ring and the upstairs person could pick up.