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A blue box is an electronic device that is used to emulate the tones generated by a telephone operator’s dialing console for switching long distance calls. The blue box was a hacking tool prolifically used in the 1960s and 1970s to enable users to route their own long-distance calls by emulating the in-band signaling mechanism used in the control of switching in long distance telephone systems.
The blue box emulated the tones generated by a switching console, usually in the 2400 and 2600 Hz range. These tones were used for controlling the switching of long distance calls.
The blue box was invented because of two crucial information relases. The first was an article published in the Bell System Technical Journal in November of 1954. It described in detail the process of routing telephone calls over a trunk line with the current signaling system of the time, the R1. The article explained the basic processes and the signals used in the inter-office trunking system, but it was not useful by itself. The other half of the information came six years later in November of 1960, when the journal published an article titled "Signaling Systems for Control of Telephone Switching," which included the frequencies used for the digits being used for actual routing codes. With both halves of the equation complete, anyone with a proper knowledge of electronics could create a device that could exploit the telephony system; the result was the blue box.
The box worked by producing tones that were used by the telephone switching system for switching long distance calls. Once a long distance call is made, the box could be used to get into operator mode and route the call to anywhere free of charge.
It was also very difficult to trace these calls, so the box quickly became popular with undesirable elements and the practice of phone phreaking immediately took off.