Line not cancelled and still being billed!!

On the 25th August 2015 I called in to cancel one of my contract lines. I was given a reference number and was told the line would be cancelled on the 25th September 2015 and that I would not be billed thereafter. Imagine my surprise when I call in today to double check if this has gone through and I find out the line is still active, I am being billed for it and the cancellation has not been processed because for some reason MTN cannot migrate the number to prepaid. Firstly, I DID NOT ASK FOR IT TO BE MIGRATED TO PREPAID I ASKED FOR IT TO BE CANCELLED. Secondly, I have paid in full on the 25th September for all charges on the number, therefore I see no reason why the cancellation was not processed. This line is not in contract anymore. I EXPECT THIS LINE CANCELLED IMMEDIATELY AND NO CHARGES BILLED TO ME FROM THE 25TH SEPTEMBER TO DATE.