Handing People over to Debt Collectors without knowing you owe them Money!

We had a Fibre Contract with Cell C. We cancelled for end of December. I received a mail confirming the cancellation and that my "clawback fee for your account is R0". So I assume everyone is happy after my last month money went off on a debit order for the last month. Then in February I started getting calls from Debt Collection company I owe Cell C cancellation money. Never did I receive any communication or a statement showing that I owe them money or following up from Cell C side asking for this money, I never new I owed Cell C any money. I received the statement from the Debt Collectors company for the first time after requesting the proof of this. And now it gets more and more every time they call me for the money or communicate with me asking when I will pay. I have mailed Cell C requesting the proof of asking for this money from me and why does the mail say I owe nothing but they still charge me without my knowledge. But no feedback as yet and once again this morning I receive a call from the Debt Collectors again. So the bill will go up once again. I do not know what to do anymore as Cell C refuse to come back, to me so we can sort this out.