Mobile Telephone Networks [MTN] South Africa – Incorrect billing

I had a month to month agreement with Supersonic for a data package, the payments were received by Supersonic, I received data for roughly 2-3 months and thereafter the service stopped. When I called to query this I was advised that the payments were being allocated to old account I had with MTN which I cancelled some time back before registering for the new account with MTN. They billed me for data that I did not receive and now Supsersonic have handed me over to a debt collecting agency that is harassing me for payments but I did not receive a service from Supersonic (MTN)