Overbilled for 'Data' I do not know of

I have an MTN mobile contract for which they are meant to debit R550. For the past 2 months - April I was debited R1500 I went to query it and I was told that I went over the limit with data usage, which is strange because I buy data whenever their data which is part of the phone package has ran out. I am not aware that I can use data when I am out of data, I did not reverse the R1500 and accepted their explanation. The second debit this month was for 1300 which I also queried in 3 different stores as their call centre is not working.. All the stores give me different answers as to why I am being billed R1300 instead of R550. I have reversed this amount and made an EFT transfer of R550 - I have a case number for which I have not received any feedback on.