We have them in our cars, the only on-going cost $9 per month for the sim-card, the other one no cost unless calls are made .
It uses GPS to lacate the car and GSM to call you, if it moves, someone opens the door, starts it , cuts the main battery or even low battery ( bellow a pre set volt value).
Much better system that QT as it has unlimited cover as long as it is above ground ..