Never received credit for returned faulty instrume

About 9 months ago I purchased a cordless Alcatel phone from the Centurion branch. I asked that it be put on my account over 12 months. After two weeks I returned the instrument as it was faulty. I handed it in at the cashier, as it is where I bought it. Now, after going through my bill, I see it was never credited. Each month R24 is added to my bill. I would like all the payments to be reversed and the instrument taken off my name. It is not my fault that the staff did not follow correct procedures