A vehicle tracking system includes a vehicle tracking unit and a monitoring station communicating therewith. The vehicle tracking unit may include a vehicle position determining device, a wireless communications device, and a controller connected to the wireless communications device and the vehicle position determining device. More particularly, the controller may have a plurality of different controller states and respond differently to a same message from the monitoring station at different times depending upon the controller state at a given time. Accordingly, a number of codes or messages used by the system can be conserved.