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January 24, 2014



Phil, it was common knowledge that the original Touch-Tone dials could be easily modified so that the 3-6-9-# keys would produce the A-B-C-D tones at the flick of a switch. The early Touch-Tone dials used a single-transistor oscillator circuit. The two tones were generated with two large ferrite-core inductors with a tuning slug in the center. One inductor generated the row frequencies, the other the column frequencies. There were taps on each inductor that the keypad switched in various combinations to produce the tones. Western Electric thoughtfully provided a 5th tap on the column coil for the A-B-C-D tones. It was trivial to cut the circuit trace leading to the 4th column and wire in a SPDT switch to select between the two taps to enable the A-B-C-D tones. Enterprising phreaks wound there own coils to turn a touch-tone pad into a blue box, although this was a considerably more difficult hack, as MF tones did not work on a row-column basis.

Those old Touch-Tone dials would go out of tune after a few years and no longer break dial tone. They could easily be re-tuned by adjusting the slug in the center of the two coils to bring the frequencies back in-spec.

Later, specialty ICs were used to manufacture Touch-Tone dials. The same trick was possible with the new ICs, which all had an unused pin for the fourth column tones.

The phreaks used the A-B-C-D tones to take over the ACD (Automatic Call Distributor) unit that distributed incoming calls to information operators. You can read about it at:


The phreak could use the extra tones to put the ACD in test mode. He could then intercept calls to information operators. You could also monitor all calls to a specific operator, a function used by supervisors.

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Just Released in Paperback

The definitive history of the phone phreaks, available in hardback, paperback, and ebook forms from Amazon | Barnes & Noble | IndieBound | Books-A-Million or wherever fine books are sold.