Charged for a account that was terminated

I terminated my 3G/Internet account Mid-November 2015 and paid the penalty fee due to already pathetic service! I received an invoice that R850 had been debited from my bank account at the end of December!!!. Phoned Neotel numerous times yesterday (6 Jan 2016) to query it and there was no answer. Eventually got through to a lady in customer services a few moments ago, was put on hold for 25 minutes just to be told that she doesn't know what the charge was for. Is there anybody who could assist? Gatvol