MTN Breach of Contract

Hi, I singed a top up contract with MTN with a fixed rate, now MTN is deducting a different amount each month. I only noticed this when a few of my debit order came back and said there is not money in my bank, resulting in R400.00 banking cost. In many attempts to sort out my account, each time the lady gives me the same feedback: \ When i asked to see the supervisor or the manager they say she is busy and going to say the same."they have queried the account and is waiting for feedback.\"" They have not called me once. I have tried to phone MTN self service 808"