Over charged/increased premium

I’ve been a loyal customer of MTN since 2008,I have two contracts with MTN &I recently renewed my other contract,this 15th November I was surprised when they say I have to pay R1643.50 while my two contracts is for R399 plus R569,when I go to the store and calls 135 no1 is able to give me clear answer,the other consultant said the contract i renewed I have to pay 1039,87 not R569 and that it was not on special when I apply for it,I tried to tell her that I have my contract documents which state clearly the amount I have to pay and she kept on arguing with me very unprofessional.At the store they could not explain coz they where suprised like me,the other consultant said it’s a once off prorata so when i asked her how can i have a prorata of R675 while my monthly installment is R569 she started insulting me saying I dnt know or understand anything with regard to proratas she’s tired of explaining.Till today I have not payed for my November bill since no1 is able to clarify me they keeps on telling me different stories.I regret the day i renewed my contract and being a loyal customer of MTN for all those years ,MTN brought nothing but confusion and frustration in my life.