Customer FAQs Discover answers to the most frequently asked questions from ADT customers with the click of a button. 1. How does my ADT alarm system work? You alarm system may be armed in either Away or Stay mode. Once the system is activated, each monitored zone has the ability to recognize a violation, i.e., a door or window opened or a motion sensor crossed. Once a zone has been violated, the alarm system communicates with ADT by transmitting a signal through a telephone line or cellular backup, if one is equipped on the system. Once the signal has been received by our ADT monitoring centers, the alarm system will wait to send another message if another zone is tripped. Entering a valid user code and disarming the system will end this process. Watch this video to learn more about how to use your ADT home security system.