A backup security system for the telephone company lines of a protected home located in a neighborhood in which all the homes have a.c. supply lines connected to a common power transformer. The protected home, in addition to being provided with a conventional security system, is provided with a line-cut monitor, a PWM transmitter, and a conflict avoidance circuit. When the line-cut monitor senses that the telephone company lines for the protected home have been cut by an intruder, it provided a signal for gating an alarm signal from the conventional security system to the PWM transmitter which impresses the alarm signal as high frequency signals on the a.c. supply lines in the neighborhood. A neighboring home in the neighborhood is provided with a PWM receiver, telephone company lines and an automatic telelphone communicator. The PWM receiver in the neighboring home responds to the high frequency signals on the a.c. supply lines and provides a digital signal to the automatic telephone communicator causing it to seize the telephone company lines for the neighboring home and send a message informing a central station that an intruder has broken into the protected home. The conflict avoidance circuit in the protected home enables the PWM transmitter therein to impress high frequency signals on the a.c. supply lines in the neighborhood only when there is an absence of any other high frequency signals on the a.c. supply lines in the neighborhood.