Unauthorised Debit orders

Cell C only has authority to debit my account once and they have taken off 5 debit orders random days of the month even after I have paid them which resulted in R100 unpaid fees and I would like my R500 back. I phoned in and somehow I didn’t pass the security check as if I don’t know my own information. Cell C only has a debit order mandate to debit my account once a month and not multiple times a month.