Blue box

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.

This article is about the phone phreaking tool. For other uses, see Blue box (disambiguation).

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.

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