Cell C *****ulently handing over paid up accounts to debt collectors

I am highly disappointed in Cell C, I had a contract with them for years, at the end of the contract I tried to renew we couldn't agree which led me opting not to renew at the end of my contract. I converted my number to prepaid

I sent them proof of my last payment at the end of the contract after they upgraded my package without any consent. I called them and we resolved that they were wrong. I have remaind on prepaid till now

2 years after the end of my contract I am getting harrased by debt collectors claiming I owe cell c. I was told it was for cancellation fee. I explained as far as my knowledge is concerned my contract ended and I owe Cell C nothing because I didn't cancel however the contract ended when the term ended

There was no communication from Cell C and only now I am having my credit score affected by their incompetency