The Mace Wireless Home Security System consists of a main control unit with a 105 dB alarm, one door/window sensor, one motion detector sensor and a remote control with panic button. The system can accommodate up to 14 sensors for expansion. You program your cell phone and additional emergency contact phone numbers into the control unit. If the alarm is triggered while you're away, the control unit will dial the first number of the programmed list. The person answering the call will hear a prerecorded message and have the option to listen in on the room, broadcast their voice through the control unit, or disarm the system. You can also install the control unit and use the remote panic button to notify someone in case of an emergency. This is an ideal use for elderly people who live alone.