Messed up credit report and incorrect deductions

Cell C has messed up my credit record. I only used their service for a month , the contract was supposed to be month to month. They deducted twice the agreed amount and I reversed the payments and paid the correct amount, they continued adding charges and have somehow gotten to the conclusion that I owe them R13 000. After numerous calls, store and head office visits , I am told that the matter is with finance who will provide feedback. I am trying to apply for a bond and it is hindering me which in putting my family in a terrible position.