The Telephone Line Manager is a powered ancillary device for monitoring telephone line voltage. The device is completely isolated electrically from the monitored line by optical coupling. It repetitively interrogates the telephone voltage to determine if the voltage exceeds the preset minimum threshold. When the voltage falls below this minimum for more than the selected delay interval, the D.P.D.T. relay switches, the alarm output contacts change state, and the alarm LED lights. A separate 12 vdc power supply is required to operate this device.