MTN Contract

I have 2 contracts, the one I pay R499 and the other one which is a sim contract I pay R199. The sim contract expired now in July and when I checked on how much I need to pay in September I was shocked to see it was R319 for the sim contraxt which have expired already. After numerous calls to MTN , no one can tell me why it changed to that amount.Im also not aware of giving MTN permission to go on another contract. On the 26th August I went into a MTN shop to go on a 6 month contract where I'll only pay R179, but yet when I checked on what will go off on the 1 Oct its yet again R309.Im so frustrated as no one at at MTN can tell me why. Its like they just dont care taking from the poor.