blue box.

About 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. 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".