Cellc's worst service ever

Cellc : Worst service ever, I have a cellphone contract with cellc so in the past months they have been changing debit amount and debits whenever they want to, the agreed amount is R319. 00 and the agreed debit date is the 20th but they have been debiting different amounts the highest was R708. 00 they even debit on the 18th and charge me R70 for a failed debit when 18th is not my pay day and not the agreed debit date. I reached out to them they said they investigating up to date but they are continuing with this behavior.