A coin box monitoring circuit for use in pay telephones, providing coin detection, coin volume totalization, and full coin box signaling to the telephone central office. An included coin chute assembly has three channels that cause nickels, dimes, and quarters to pass through specific locations within each channel. Light sources and light detectors detect the passage of coins. A microprocessor is utilized to control the light sources, monitor the detectors, and volumetrically count the coins passing through the coin chute. On reaching a full condition, a microprocessor then controls a relay to signal that the predetermined coin box level has been reached.