Importance

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Abstract

A circuit for preventing a telephone set from being used without approval, the telephone set including a handset, telephone lines coupled to the handset, a dial pulse signal generator and hookswitches for coupling the telephone lines to the handset and for cooperating with the handset. A circuit is provided for converting the dial pulse signals into a binary code, a prohibit circuit is provided for generating a prohibit signal which prohibits a telephone call when the binary codes corresponding to prohibited dial numbers are input thereto, and an unlock circuit is provided for generating during a predetermined period a control signal which allows a telephone call when the binary codes corresponding to secret dial numbers are input thereto. A transfer switching means is provided for selectively coupling the converting circuit to either the prohibit circuit or to the unlock circuit and for cooperating with the hookswitches. A gate circuit is provided for preventing the passing of the prohibit signal of the prohibit circuit in response to the control signal of the unlock circuit, and a switch is provided for breaking the telephone lines in response to the prohibit signal from the gate circuit. A reset signal is generated to the prohibit circuit when the hookswitches are closed, whereby the prohibit circuit is reset to its initial condition.

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First Claim