Clear box (phreaking)

A clear box is an amplifier used by phreaks to use post-pay pay phones without paying. In some locations, especially rural areas in the United States and Canada, pay phones were configured for "post-pay" operation. In this mode, the handset microphone is muted until payment is made. The user of a post-pay pay phone would dial first, wait until the called party answered, and at that point the user would be prompted to insert the coins. Upon receiving the correct payment for the call, the pay phone would then connect the microphone and allow the caller to speak. An artifact of this scheme was that the called party usually heard the ACTS tones produced by the pay phone upon coin deposit.

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