Constantly being charged more than my contract

I have 2 contracts with MTN, one for myself and the other for my wife. I had recently upgraded my wife's contract and had requested a cap on the amount. The consultant at PMB Midlands Mall said that they include a limit of R1 so that I should never pay more than R1 more than the contract amount. The capping amount has not worked at all, every month I received a bill for at least R100 more than the contract value and I am tired of contacting customer services to correct these issues. I had even posted a complaint on hellopeter as customer services takes forever to answer as I had once waited 20 minutes and nobody from MTN got back to me on that. I am sure that MTN is in breach of the contract that I have as they are not allowed to bill more than the signed agreement has stipulated