Double Billing and unethocal billing practises

My debit order with Telkom is scheduled to go off every month at the end of the month however in the Month of September this never happened.They decided to debit my account on the 23 of October when there is no funds in my account.This threw my account in arrears and they disconnected my line and handed my account to the collections department.They said that once i can pay the outstanding amount that they never debited of R504 they will activate my line so i paid the balance although it was their mistake.The line got activated and that issue was sorted.On the 30/11/2015 they double debited my account an amount of R1013.My contract states that the amount that will be debited is R504 and they debited R1013 so they debited an extra amount of R509.When I called them they gave me endless excuses as to why they cannot refund me and they showed no remorse for making the mistake of not debiting my account.I am R509 short on my account and i need my money back.Hoping you can assist me with that.I also need my account to be taken out of collections as this has greatly affected my credit record.