MTN debiting my account with a higher amount every month

Both my partner and I upgraded our contracts to 18 month sim only data contract which costs R299. After a month of the contract being in place the bill went from R299 to R329. This has been ongoing fir both of us. I went to MTN to query this and I it was fixed for the month of March and it went back up the month after. I have tried to call mtn customer care but it's almost impossible. I don't understand how can mtn be taking a higher amount if ita not what is it the contract.