Payment wanted for defective unit

Since joining Cartrack ive been a happy client, i love the reports and features of the app. All of the sudden it no longer updated. I called to test and they said it is working fine. I got a new phone amd installed the app and it was working fine again. I then didn't go on it at much. One day i get a sms saying to call a number to test my unit. I call and the unit does not work. I asked if i could do it on my off day they said a agent will call me to arrange a replacement unit. Ive been paying manually as i had some issues with my bank and changed accounts. Months went pass and no appointment was booked. Cartrack sends me sms's to say that my account is in arrears. I spoke again to a collections agent and i said i will not pay until my unit is fixed why should i pay for something that is not working? Again they said they will get someone to call me and still to this date no one has called me to book a replacement unit. I constantly get sms' asking me to pay but no call to book a new unit. I will not pay for something that is not working i will not backpay for something that was not working. I want the unit replaced and i will continue payment as usual