MTN BLOCKS MY PHONE WHILE OWING ME MORE THAN R4000

I had 2 contracts at MTN on my name. The one was for me and the other for my son. In 2013 the contract on my son's phone expired. I called in and cancelled that contract. I extended my own contract. Two years later I discovered they were still deducting the installment of my son's contract. I called MTN and they confirmed that the contract was cancelled. They admitted they owed me more than R4000 and would refund me. I asked them to deduct my installment on my current contract and pay out the balance to me. I refused to pay my installment and now they have blocked my phone and handed me over to their legal department. Despite numerous calls, nobody seems to want to help me and the past 2 months they strike and nobody even answers the phone. I'm desperate to sort this out as I'm using my phone for work purposes. My own contract expires tomorrow and I'm definitely going to cancel everything at MTN and never deal with them again!!!!