Importance

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Abstract

A device is provided to prevent an unauthorized telephone unit from initiating a telephone call, where the unauthorized telephone unit is connected between the central office of the telephone company and the authorized telephone units of the user. The device includes a first detector for detecting a DC current within the lines, and a second detector for detecting the voltage across the lines which lead from the telephone office to the authorized phones. A third detector detects a dual tone multifrequency dialing signal on the line, and a fourth detector detects pulse dialing, and a logic is responsive to all four detectors. When a dialing attempt is made from an unauthorized telephone unit, the first detector will not detect a DC current in the lines to the authorized telephone units, the second detector will detect a drop in voltage across the ring and tip lines, and either the third or fourth detector will detect that a dialing effort is being made. Where an unauthorized dialing is detected, the logic switches a blocking device between the lines to block the unauthorized dialing efforts.

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First Claim