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Concept
Blue box
Summary
A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. This allowed an illicit user, referred to as a "phreaker", to place long-distance calls, without using the network's user facilities, that would be billed to another number or dismissed entirely as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.
First developed in the 1960s and used by a small phreaker community, the introduction of low-cost microelectronics in the early 1970s greatly simplified these devices to the point where they could be constructed by anyone reasonably competent with a soldering iron or breadboard construction. Soon after, models of relatively low quality were being offered fully assembled, but these often required tinkering by the user to remain operational.
The long-distance network became digitized, replacing the audio call-control tones with out-of-band signaling methods in the form of common-channel signaling (CCS) carried digitally on a separate channel inaccessible to the telephone user. The audio-tone-based blue boxes were of limited use by the 1980s, and of little use today.
Local calling had been increasingly automated through the first half of the 20th century, but long-distance calling still required operator intervention. Automation was deemed essential by AT&T. By the 1940s they had developed a system that used audible tones played over the long-distance lines to control network connections. Tone pairs, referred to as multi-frequency (MF) signals, were assigned to the digits used for telephone numbers. A different, single tone, referred to as single frequency (SF), was used as a line status signal.
This new system allowed the telephone network to be increasingly automated by deploying the dialers and tone generators on an as-required basis, starting with the busier exchanges.
Official source
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