Bad Billing-Why should I pay for their mistake.

On the 02.03.2015, I had a debit from my bank account for the amount of R 2 388.23, I contacted Vodacom on the 16.03.2015 and the agent advised that I should contact my bank as they had no record on this. (REALLY!?)After reversing the payment, I received a statement stating that there was an outstanding amount of R 2 388.23, on the 11.03.2015 I emailed to ask what the amount was for - with no reply I then emailed 13.03.2015, I eventually received a reply with my statement which I ALREADY HAD but needed to know what my bill was for as I have a capped line. With no feedback.On Monday 25 May, I called in to the call center and asked the question again... I was advised that I ran out of data in January and was then billed out of bundle rates..BUT WHY... I have a capped account which has always been a capped line and this never happened before! Later that day I received a call from, accounts department requesting payment, I explain my story she said I should contact customer service and their data dep. which I did but they advised I should contact collection.Why am I being charged out of bundle rates on a capped line.<br> Why am I being charged out of bundle rates on a capped line.