Contract closed but still gets billed

I paid off my Vodacom contract last year and Vodacom started phoning me a week ago stating that I owe them money. I gave them proof of payment and Vodacom Tygervalley advised me that the balance is R0.00 and the contract is cancelled. Vodacom is currently harrassing me with their attorneys, deamanding that I pay R1108.46. I expained to them that the employee at Vodacom Tygervalley did not advise me that I required a paid up letter. He just advised me that the contract is paid up. Now they are making it my fault. This is unneceptable and someone must take responsibility for this and I am not going pay it. This fault is on Vodacom's side because clearly someone did not do their job properly.