instalment increased

I've noticed my instalment went up without being notified. When I query the consultant told me is because something to do the bill that have increased on their site so when it happens they automatically increase their clients instalment. MTN listen here I signed a contract that said I'm going to pay R449 a month for the period of 24 month, I never miss any instalment and today you telling me you increasing the instalment because your whatever increased theirs. I dont give a damn about your problem but my contract clearly says R449 a month so please fix it and be advise that this is the last contract im involve with MTN. You didn't tell me the instalment may go up you just said is for 24 month and you must pay R449 a month!