Bills received on supposedly capped contracts

I took out a phone contract with the MTN shop in Eastgate and a ****le contract with the MTN in Nicolway. I was very clear that I required my contracts to be capped - I have witnesses for this and enquired in both shops atleast two times to make sure the contracts were capped. I do not have the financial capability for an uncapped contract. Further, nowhere on my contracts does it state capped/uncapped - this issue needs to be addressed. However, two and a bit months into my phone contract and a little more than a month into my ****le contract, I received a bill totalling R4564.66. When I enquired, the sales assistant said it was because my contracts were uncapped. How did they suddenly become uncapped? I believe it was a fault on the system, as this happened to me, my flatmate and another friend, who all have new contracts with MTN (taken out in Feb/March 2015). It is not my responsibility to pay this bill as this was not a mistake on my side. How do I resolve this issue? I have logged queries from both Nicolway and Eastgate to head office, as well as logged customer complaints and queries on the MTN website, but to no avail. This bill has now increased to R5515.25. Very unhappy.