Toll fraud eliminator for telephone systems

Abstract

A tone detector is connected or otherwise associated with an outgoing telephone trunk line adapted to be included in a toll path. One or more frequency or tone signals are sent over the trunk line for signaling and control during a toll call. If these tone signals appear at a point on the trunk line where the toll call remains under the control of d.c. signals, they must be fraudulent since there is no preceding equipment which is adapted to send valid tone signals. Therefore, a tone detector may be connected to that point in order to drop or otherwise block any calls during which the fraudulent tone signals appear. The toll fraud eliminator is particularly -- but not exclusively -- useful for preventing employment of a 'blue box' during toll calls. The tone detector uses two multivibrators which operate at an upper and a lower threshold frequency respectively. The half cycles of a given polarity in the single frequency tone must recur fast enough to continuously reset the multivibrator running at the lower threshold frequency, but not fast enough to continuously reset the multivibrator running at the upper threshold frequency.