Cell c worst service ever!!!

I made full payment to cell c in April 2019 but I still get calls from NDS credit saying I owe cell c. I called cell c call center when told me to call their legal department, the problem is that they are always unavailable and when I call the call center agent she just told me she doesn't know when they will be back. I keep calling everyday and no response meanwhile nds keeps calling me for the same thing.