Faulty tracking unit

In March I called and asked about my tracking unit, then someone was sent to check my unit only to find out the unit wasn't working. I called them on the 20th of March trying to find out when exactly this unit had a fault so they can refund my money since I was paying for faulty device. I was promised a call by the supervisor and till date no one called, instead they took my account to the accounts department. Till today no one has called, they ignore me when I raise the faulty device issue but want to force me to pay.