> one-transistor DTMF generator
I did not know that. Your post made my day.
I was not quite old or connected enough to become a phone phreak, but I did do a little experiment to see if audio could be sent to a caller without picking up an incoming call. I put the high impedance side of an audio output transformer on the line and put audio into the low impedance side, then drove up to the gas station and called home. I could hear the audio. The high side of the transformer did not take the line off hook, but with audio going out at a voltage competitie with the ring voltage, I could hear the sound on the calling end.
Also I hope every kid who was interested in the phone system figured out how to dial out without using the dial.
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.