Bad Customer Service

I received an sms in January regarding an outstanding bill from Lancet. I called the call centre and requested the agent send me an invoice via e-mail with the correct amount and banking details. The amount due on the invoice was R302 so we paid the account the same day and followed up a week later. I received a settlement invoice with a balance of R0. Last week Friday I got a nasty sms saying I owing R56.10 and if I do not pay I would be handed over. I called the call centre irritated as hell, how can I receive these sms when I have a letter stating my balance is R0. As per usual the managers are never available to take your calls and the promises to call me back never materialises. The agents are so ***********, they can never assist in resolving your queries. I received another sms yesterday with the same *******. Fix your account systems and stop sending threatening sms that hold no ground