Expired contract still billed for

I had a mobile contract with Vodacom many years ago. When the contract expired it automatically changed to a month-to-month basis - there are recorded conversations with Vodacom where the agent confirmed this. Regardless, Vodacom continued to bill me (without my knowledge) for a contract - which I of course did not pay as this billing was in error and *******. Since then the account has bounced from one debt collection agency to the next, each one throwing it out once they do their due diligence and discover the error is on Vodacom's side. I have just found out this same account has been listed on my credit record, and, according to the current debt collectors - who've put it on hold - cannot be removed by anyone but Vodacom themselves. I have been trying and trying to get through to Vodacom Legal to sort this out and have them remove the listing from my credit record, but I am being repeatedly cut-off, redirected, or told they cannot help me. Obviously the negative record is affecting my credit rating and thus my ability to apply for credit. I need this dispute settled immediately and the listing removed.