different billing amount monthly

Cell C has been debiting me different amounts monthly, yet when i upgraded my contract was indicated that i will be paying R346.95 excluding vat and including i will be paying R399 , and i've also indicated that the debit order should go off the 26th but now in August was charged R399 with additional R70 rands of failed debit order, In July was debited R727.99 with R70 failed debit order , June was debited R398.99 with additional R70 debit order , In May debited R328.99 then april R591. I called consultant this morning who had no explanation or any idea spend 15 mins on a call with no explanation only to be given an email to email another department yet in the beginning she indicated that she will help regarding billing. I am tired of cell c this is my 3rd upgrade with them but to my surprise am still told i can still upgrade yet i have already upgraded .