A method and apparatus for measuring signals conveyed by a varying the voltage polarity across the tip and ring terminals in a telephone system is disclosed. The invention provides a composite circuit including a first stage in the form of a differential amplifier, a second stage in the form of a low pass filter, and a third stage in the form of a Schmidt trigger. The invention is capable of discerning an intentional polarity reversal telephony signal by eliminating transmission noise, as well as false triggering due to a high frequency (relative to a defined threshold) ring signal.