Phreaking Out Ma Bell - IEEE Spectrum:
Now that New York knows the number you want to call, it makes the local connection and the directory assistance operator’s telephone starts to ring. Up until now everything that has happened has been perfectly normal, just like Ma Bell intended. But now you, using Barclay’s hack, insert yourself into the process. Before the operator can answer, you—naughty you—hold a speaker up to your phone’s mouthpiece and play your own 2600-Hz tone down the line for a second.
It is loud and pure, and it sounds like this: “Bleeeeeeep.”
Seattle isn’t paying any attention to this, but the switching machine in New York sure is. New York hears your 2600-Hz tone loud and clear and thinks that the Seattle switching machine sent it. And since this tone indicates the trunk line is idle, New York figures that Seattle is done using that trunk line, probably because you hung up. New York disconnects the call to the directory assistance operator—maybe before she’s even answered.
But now you stop sending your tone. When you stop sending 2600 Hz, the long-distance switching equipment in New York City now thinks that Seattle wants to make another call. Just like before, New York sends a wink back to Seattle to say that it’s ready for a new call. Due to the nature of the circuitry involved, the wink has a bright, metallic, ringing quality to it. It sounds like this: “Kerchink!”