Pay Phone

A pay phone sent a series of tones when a coin was inserted to let the equipment at the phone company know that a coin and what kind of coin had been inserted.

Back in the day, phreaking was a common way to steal payphone service by faking those tones.

Maybe my sample set is skewed, because I knew a lot of people in the telcom industry. In high school a modified radio shack speed dialer was often used to access the payphone on campus or at the mall for free. The captain crunch whistle pre-dates me, so I don't know how common that was. By the time I learned about blue and black boxes they didn't work.

YouTube surprisingly has a video on the radio shack dialer.

https://www.youtube.com/watch?v=AXZMgHKhefk

My roommate in college did a lot of hacked conf calls, wasn't my thing. Most were exactly how you'd imagine a phone call with a bunch of anonymous weirdos would be. Several people talking at once, all trying to say outrageous things.

Prior to about 1970, pay phones had actual bells in them that inserted coins struck on their way to the coin box. When the initial period (often three minutes) was up, the operator would ask you for more money and listen to the different bells to hear what coins you'd deposited.

Modern pay phones, including all the ones with Touch-Tone pads, had tone generators instead of actual bells. Those tones—which were different from the ones generated by Touch-Tone pads or in-band signaling keypads—could be interpreted by a central office computer as well as a human operator. The red box was a way of stealing service by emulating the tones without depositing any coins.