Abstract: A coin tone validator (CTV) is located in a local central office and is connected between a pay telephone and a switching system. The CTV monitors DC loop current changes using opto-couplers inserted into a local loop in addition to measuring standard 1700 Hz and 2200 Hz tone frequencies generated when a coin is inserted into the paystation. A comparison of the two components, the current changes and the frequencies, is performed to determine a timing and magnitude relationship. If the frequency component is misaligned within a specified time frame or the magnitude is not sufficient, then the tones received or the current change sensed are dismissed as interference. The CTV prevents users of pay telephones from fraudulently generating coin tones that are acoustically coupled into the handset of a paystation from a calling station at any time during a call.