Incorrect Billing on a top up contract

I signed for a Mychoice Top up R25 contract. In June I was billed for the amount of +/- R900. I went to the store again to find out what the charges are for. The lady that assisted me said that there was an error on the system and that she will log a request to fix it. She advised me that they MTN is on strike and it will take a long time to reverse the money. She also advised me that the system shows that I am capped and she does not know why I am being billed. I then went to the bank and reversed the funds as I had other debit orders due on my account. I was billed again at the end of July for an amount of R1098.00. I again went to the MTN store to find out why it was not sorted out. She could not even provide me with a timeline. I told her that its urgent and she advised me that I should go an reverse the money at the bank. I also asked both the call centre and the store what the amount is that I need to pay and they cannot tell me. My contract amount is R99 per month. . From the time I received this phone I bought airtime from my FNB telephone banking when I needed to make a call. If my airtime is finish I buy my own airtime. Why am I being billed when I am capped? Please help