Blue box

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

Blue box designed and built by Steve Wozniak and sold by Steve Jobs before they founded Apple. Displayed at the Powerhouse Museum, from the collection of the Computer History Museum

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. During that period, charges for long-distance calling were commonplace and could be significant, depending on the time, duration and destination of the call. A blue box device allowed illicit users to circumvent these charges, placing long-distance calls that would be billed to another number or dismissed entirely by the telecom company's billing system as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network, a practice called "phreaking".

First developed in the 1960s and used by a small phreaker community, these devices were greatly simplified by the introduction of low-cost microelectronics in the early 1970s to the point where they could be constructed by anyone reasonably competent with a soldering iron or breadboard construction. Soon fully-assembled models were being offered, although these were of relatively low quality and often required tinkering by the user to remain operational.

Over time, as the long-distance network became digitized, the audio call-control tones were replaced with out-of-band signaling methods in the form of common-channel signaling (CCS). Control signals were now carried digitally on a separate channel inaccessible to the telephone user. This development limited the usefulness of audio-tone-based blue boxes by the 1980s, and they are of little or no use today.