Cell C account handed over to NDS

My Cell C account was handed over to a company called Nds Credit Management.. This morning Cell C contacted me to let me know that the account with them has been closed. But Nds Credit Management keeps on telling me that I owe them R1100 where my account with CELLC was Less than R3000 and I have been paying monthly for over 3years the amount paid is already well over R3000. I even have the proof that it's been paid we keep on sending it to them but no one has ever received nothing according to them. HOW MUCH MUST I PAY FOR HOW LONG!?!?