Unfair Billing

I have a tracker with Cartrack.On the 29th of November I received a call from them informing me that my unit was faulty and it needed to be replaced.I asked if I was going to be charged for that and the consultant said No.Surprisingly on the 22nd of December I received a call from Cartrack accounts department saying that I had been billed.This is not only unfair practice but it shows serious levels of misleading a client.Cartrack records calls and they surely have a call in which I clearly asked if I was going to be charged and was told no.Such kind of behaviour cannot be right and it should be stopped.