Unhappy customer

Vodacom has sent me a letter of demand because I reversed a R120 transaction in January because the user of the sim card took out an airtime advance which they settle by buying airtime but for some reason when vodacom debited my account they also included that airtime advance so 6 months later they sent me a letter of demand totaling R4200 and when I request for a statement detailing how the amount got to 4200 no one is responding they threaten me with handing me over to debt collectors and listing my name at the credit bureau ....ever since I have heard a contract with them I have always asked for a statement they fail to send it always asking me to confirm email address but when they sent me the letter for some reason they managed fine with the email ...they are bullies