Billing incorrect after phone upgrade

Since I have upgraded my daughter's phone in February, I get billed 2X and 3X more than what the contract is, We upgraded from BB BIS to uChoose Flexi 110. When I signed the contract the monthly payment came to less than R500. Yet since March the bill various between R900 & R2500. I have logged another request on the website (M***) on the 3rd of Sept. Still waiting feedback. When I dail *111# to check my data bundles, it still show that I'm on BB BIS.