Cancellation dispute

I had two contracts with you and my terms ended my last payment was December 2020.. now I keep getting calls from nu debt telling me you handed me over to them due to me not making payment... My frustration is you waited 3 years without any communication, your collection team didn't call me all of the sudden I'm in debt I owe you.. how is that even possible.. and by law you didn't follow popia act.. Iv been calling telkom and they tell me that I don't owe anything yet at the same time I get calls from nu debt telling me they putting me on itc😠😠😠😠 how ???? I NEED EXPLANATION