2600 Hz

A blue box is an electronic device that simulated an operator by generating the same tones employed by a telephone operator's dialing console to, for example, switch long-distance calls. A blue box is a tool that emerged in the 1960s and '70s; it allowed users to route their own calls by emulating the in-band signaling mechanism that then controlled switching in long distance dialing systems. The most typical use of a blue box was to place free telephone calls. A related device, the black box, enabled one to receive calls which were free to the caller. The blue box no longer works in most Western nations, as modern switching systems do not use in-band signaling. Instead, signaling occurs on an out-of-band channel which cannot be accessed from the line the caller is using, a system called Common Channel Interoffice Signaling or CCIS.

In November 1954, the Bell System Technical Journal published an article titled "In-Band Single-Frequency Signaling" which described the process used for routing telephone calls over trunk lines with the then-current signaling system, R1. The article described the basics of the inter-office trunking system and the signalling used. This, while handy, could not be used in and of itself, as the frequencies used for the Multi-Frequency, or "MF", tones were not published in this article.

In November, 1960, the other half of the equation was revealed by the Bell System Technical Journal: another article titled "Signaling Systems for Control of Telephone Switching" was published containing the frequencies used for the digits that were used for the actual routing codes With these two items of information, the phone system was at the disposal of anyone with a cursory knowledge of electronics.

However, contrary to numerous stories, before finding the articles in the Bell System Technical Journal it was discovered by many, some very unintentionally and to their annoyance, that a 2600 Hz tone, used by AT&T Corporation as a steady signal to mark currently unused long-distance telephone lines, or "trunk lines", would reset those lines. Joe Engressia (known as Joybubbles) accidentally discovered it at the age of 7 by whistling (with his mouth). He and other famous phone phreaks such as "Bill from New York" and "The Glitch", trained themselves to whistle 2600 Hz to reset a trunk line. They also learned how to route phone calls by causing trunks to flash in certain patterns. At one point in the 1960s, packets of the Cap'n Crunch breakfast cereal included a free gift: a small whistle that (by coincidence) generated a 2600 Hz tone when one of the whistle's two holes was covered. The phreaker John Draper adopted his nickname "Captain Crunch" from this whistle. Others would utilize exotic birds such as canaries which are able to hit the 2600 Hz tone to the same effect.

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Wikipedia