a voltage detection circuit , in conjunction with a power supply system for ringing voltage generation in a remote telecommunications unit , monitors the operating voltage level of the remote unit and , when an undervoltage condition is detected , causes a ringing reference - signal generator to collapse the reference sine wave into a fixed state and holds it in this state for a preselected time , thereby substantially reducing the ringing - signal current load and allowing the operating voltage level to restore . the features of the invention allow for continued processing of calls in progress and for incoming call completion during an undervoltage condition . the increased reliability and integrity of the power supply system allows for extended power supply feeder distance to the remote telecommunications unit .