Downright theft!

I ported from MTN to Vodacom in July 2015. I originally transferred from a corporate contract to a private contract and do not recall ever having filled in a debit order form. I paid my account by EFT from January 2015 to July 2015. After having ported (and this being acknowledged by MTN), MTN continued billing me and started debiting my account. I contacted them as soon as I saw the debit appear on my account in August 2015 and have done so almost every month since. They keep telling me that my refund is with their billing department, but the R1,800+ that they owe has still not been repaid. I cannot get to speak to anyone senior enough to handle this matter nor have I been able to get a copy of the debit order entitling them to deduct funds from my account. I have never experienced such incompetence. My phone calls to MTN have probably cost me as much as they owe me, as each call lasts about half an hour or more.