Discussion
A toy whistle from Cap'n Crunch cereal made almost exactly the 2600-hertz tone AT&T used on its phone lines, and a phone phreaker took his nickname from it...
The toy whistle packed into Cap'n Crunch boxes in 1963 emitted a 2600-hertz tone, according to Wikipedia's article on John Draper, and Wikipedia says that is the frequency AT&T's long lines used to indicate that a trunk line was available. A cereal prize ended up speaking the phone company's own control language.
Nobody planned it.
A toy gave a person his nickname and a magazine its name.
If you build anything that listens for a signal on the same line that carries everything else, a password, a magic phrase, a tone, assume someone will find a cheap thing that says it. Look at what your system trusts just because it arrived, and move the control signals to a place your customers can't reach.
Any channel your customers can talk on is a channel someone will talk to you on. Keep your control signals off it.
Make Tech Easier says the match was entirely by accident, a tone at almost exactly 2600 hertz. Living Internet describes people using the whistle from a Cap'n Crunch box to make free long distance telephone calls, with a modified whistle.
The trick gave one man his handle. Living Internet says John Draper popularized the whistle and became known by the hacker handle Cap'n Crunch. Wikipedia adds that 2600: The Hacker Quarterly, founded in 1984, was named after the tone.