a $20 pre-paid sim has $20 credit on it, it only needs to be recharged as you run out of credit, or every 6 months, they are with telstra (best mobile coverage)
If the car is stolen, you send the car a sms via the mob #, and it will instantly respond with location, current speed etc, and you can even send commands like immobilize, lock doors etc. you also get a sms/phone call when the car has been broken into and another if it gets started.