A red box is a phreaking device that generates tones to simulate inserting coins in pay phones, thus fooling the system into completing free calls. In the United States, a nickel is represented by one tone, a dime by two, and a quarter by a set of 5 tones. Any device capable of playing back recorded sounds can potentially be used as a red box. Commonly used devices include modified Radio Shack tone dialers, personal MP3 players, and audio-recording greeting cards.
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The tones are made by playing back 1700 Hz and 2200 Hz tones together. One 66 ms tone represents a nickel. A set of 2 66 ms tones separated by 66 ms intervals represent a dime, and a quarter is represented by a set of 5 33 ms tones with 33 ms pauses.
The system that handles these tones is called the Automated Coin Toll Service, or ACTS. However, since ACTS has been phased out of service in much of the United States, combined with the integration of acoustic filters into many payphone handsets, the practice of red boxing is rarely possible anymore.
In the UK, a 1000 Hz tone for 200 ms represents a 10p coin, and 1000 Hz for 350 ms represents a 50p coin. Prior to this system, the earliest UK pay-on-answer payphones used a resistance, inserted into the loop for one or several short periods, to signal units of money inserted. Phreaks simulated these signals by unscrewing the microphone cover of the handset and inserting into the microphone circuit a resistor in parallel with a press-to-open push-button. This was hard to do inconspicuously in outdoor payphones, and was more common indoors (e.g., in student halls of residence).
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