Bad Service/ Billing/ Lack of Responsibilities

On 2015-01-10 I took out a contract cellphone for my child studying in Johannesburg, ***, from CELL C ,Queenstown. It is a Samsung G800 GalaXy 55 Mini for 24 months at R299 per month including 1Gig Data the consultant explained to me and what I requested and signed for nothing more or less. In March 2015 I received a sms from CELL C that I owed R5 124.15 so I went to the branch to enquire and the amount increased now to R12 192.20, asked how was that possible nobody was able to give me an answer after some time the consultant told me I was that I was on an open contract and they had forgotten to put on a limit for it to stop at R299. Spoke to the store manager who said she wasn't able to help me a referred me to the customer care which tells me that the mistake should be sorted by the store, I have been complaining about this issue but no feedback is being given to me not by the store or customer care. It is not my fault that the consultant was careless and forgot to do the duty that she promised me because this goes back to poor customer service, I cannot be paying for something I shouldn't according to my consumer rights. And now this issue has a lack of consideration