Billing Limit over exceeded even though capped

I am highly annoyed with the billing for my contract phone. First I had to wait ages to get the handset after doing the upgrade, now I have the problem of my bill exceeding the limit, even though it has been capped, twice. I now am having to pay an amount of R1800 or so, when in the previous months the same thing has happened. I took the contract knowing it would be within my budget, but it has been a complete nightmare. The Greenstone Mall staff Dimitri and Thabiso have been so great in trying to assist me (thank you), however why should I pay for this bill. I have been told that I need to pay the amount and it will be carried over, as I did in the months prior. That is completely unacceptable as it was CAPPED. I purchase Data of 2GB every month, to ensure I dont have this problem, but, still I get a bill that shows R1900 or R1800. I cannot understand how this bill should be my problem again, when I had DATA loaded, and, everything was CAPPED. How is MTN controlling their billing? I am annoyed and frustrated and completely dissatisfied with MTN as a whole.