Billed for a device that's not working

I have been going back and forth with Tracker regarding my faulty tracking device , I was sent to a technician who had it checked and said it was fine. I called tracker later that day and they told me they couldn't trace my car. It was checked again and at some point I received a call that said my car was in a dangerous area, while it was parked in my garage. This is just some of the incidents that I have had, which clearly indicate that the device is faulty. Due to the fact that Tracker has failed to fix their device and has instead been taking me from pillar to post, I decided to stop paying my subscription fees, as it does not make sense to pay for a service that you are not getting. The next thing I know I’m getting calls from some collections company regarding an amount I owe to Tracker. Can someone from Tracker explain to me what this amount is for? And please can I have a competent person this time around, as all the tracker consultants that I have spoken to have been telling me different stories!