unacceptable service

I have a tracker unit which i have been paying for almost 3 years and i havent benefited from it at all. i spoke to someone from tracker a few months ago with regards to my unit being faulty, they said they checked it on their side and it was fine and that i should just replace the battery to the key ring device, which i have done, i have followed the reset process of the device and still it is not working. i called them this morning, first i am told that there is no active vehicle on my name and then i was told that i should pay a call out fee of almost R400 for them to send out a technician to check what is wrong with the device, i feel this is unacceptable as i have been paying every month and it has not benefited me at all. i refuse to pay a call out fee for a device that is not working. tracker needs to either cover the call out fee and send someone to check it out and fix it or they should cancel my contract