Vodacom has handed me over for a paid up account!!

So.... I got a VERY disturbing phone call on Saturday 23/05/2015 at around noon, from a collection agency saying that I have been handed over to them in early MARCH 2015 by Vodacom, as they(Vodacom) have been trying without success to contact me regarding an outstanding amount on my account. I asked the lady caller on the phone how this was possible, as the number in question was still active, and that Vodacom debits my account every month without fail? She then become very uptight, saying she is only doing her job, and wanted to know when I was going to pay?! I ended the call and called 111, Vodacoms customer care. It was then deemed that I don't owe any money and that Legal department would be in contact with me to resolve the issue. I have now phoned back THREE times, spending on ave 15 - 30 mins per call getting NOWHERE! I am now being threatened with legal actions by MBDCS if I dont pay them!? Why does Vodacom just ignore me?