line that was not capped

On the 15 Aug 2015 I went to Cell C @ the mall @ Reds to upgrade my current Cell C contract. So i did the upgrade and also took out a new data contract seperately for 99 rand. I told the sales person to cap the line at 50 Rand, therafter i confirmed with him three times and he said that it is definitely capped. Even before I left the store I asked once again and he confirmed that it is capped. A few weeks later i checked my current balanceand got a shock to see that my current balance is R3200. I then called cell c call centre and asked them if it was capped and they said it was not. I then told them to cap the line at R50 once again. This month i received my statement and it is now R2900 . When i called the call centre they told me that it wasnt capped. This is not how you conduct business. I have been a cell c customer for many years and i havent had any issues until now. I really am furious as this wasnt my fault and it is really a lot of money. I request an urgent investigation into this matter .