CellC wrongly affecting credit score

End 2019 I renewed my cellphone contract. From the word go there was a problem with the contract. I contacted CellC repeatedly, with no feedback and the problem persisted. I told them I want to cancel the contract. I was given an amount to be paid so that I can cancel the contract. I paid that amount and moved my business to Vodacom. Now I see that on Clear Score I am listed as a non payer by CellC. And an agency is calling me time and again, stating they are acting on behalf of CellC. I phoned CellC last week and was told I don't owe CellC any money, in fact they owe me money. I have brought this under the attention of CellC many times per email. They don't come back to me or remove this thing on Clear Score. What must I do to get them to clear my (credit score) name?