UNFAIR BILLING

I have a contract with MTN. All along I have been receiving an invoice on a monthly basis, with a detailed list of all the numbers I have called, internet usage expense and so on. At the end of July my bank account was debited by approximately R 4 000 and shockingly, I never received my monthly invoice. I have been calling MTN at least 3 times a week since then, requesting the invoice but all I was told was it's not possible for them to send me the detailed invoice (itemised billing as they call it). Now I have a problem with that!! I find it rather odd that someone tells me it is all of a sudden not possible for them to show me what makes up the total amount of the money they took from my bank account. I want them to account for this or pay my money back if they can't.