Contract Cancellation

I've been struggling with MTN for over 6 months now to get them to cancel a promotional phone, sim card and contract which was sent back to them within the 10 day time frame. Each month they still collect the money for said phone and contract, and every time I phone the required call centre I get a new reference number and get told they will escalate it urgently and get back to me within a week or so. We're at the end of July now, and still no luck in cancelling this contract!