Increase monthly premium of contract

I have a top up contract with MTN and I signed the contract for an amount of R239pm, my contract ends this year December/January 2015. But I received my July 2015 statement and they just increased my premium to R241pm. Why do you sign a contract for an amount if they can just increase it when they like. I would have never taken the contract if that is the case. Why do you then call it a contract. According to law that is not allowed. I have e-mail customer care and said that it's unacceptable, that was Friday. I still have not had any response from them. I feel if they can not keep their end of the contract then I can not keep mine and feel that it would just be right to cancel the contract as I did not sign for a premium of R241. I was informed that all the MTN contract premium has been increased, that is just not right! Check your bill/statement people!