Faults on invoices not attended to

I've got two contracts on one account. The one contract started in March 2014 and the other was an upgrade in May 2014. Upon asking for Top-up contracts, as I had before the upgrade on the one contract, nothing was mentioned that it worked differently than before. However, from the first invoices in both cases, the invoices fluctuated from month to month. Numerous calls was made to customer care to fix the amount (or convert it to a top-up contract or whatever they call it now) Upon each request to fix the monthly account, customer care gives a different reason as to why it is still not fixed and then set the monthly cap. However, the invoices still fluctuate! The MTN store @ President Hyper, Krugersdorp, was very helpful and also lodged a complaint and requested explanation as to why the invoices fluctuate. Nobody responds. This is going on for more than a year now.