Timing circuit and a secure telephone jack utilizing this circuit

Abstract

A timing circuit and apparatus that utilizes this circuit. The apparatus is particularly suited for use with a telephone network interface, for substantially preventing a third party who gains access to the interface from placing an unauthorized telephone call through a network connection obtained through a jack, typically modular, located in a customer portion of the interface. Specifically, this apparatus is connected in series within the telephone company side of the interface to illustratively a ring lead of an incoming subscriber loop. In operation, the apparatus provides network access, by supplying loop current, on a timed periodic (repetitive) basis to a telephone (or other equipment through which a telephone call can be placed) plugged into the jack located within the network interface. Access is provided for a period of time that is sufficient to test loop current but is insufficient to allow the network to complete a call dialed from that telephone over this loop. However, between any two successive intervals during which the apparatus supplies loop current to the telephone, the apparatus interrupts this current for a period of time that is sufficient for a switching machine at a local central office that serves this loop to interpret the cessation of current as an on-hook condition, completely terminate any call that may be in progress for this telephone and reset the loop.