Cell C billing disaster!

I cancelled my Cell C contract on the 16th of November 2021 and it was terminated on the 16th of December 2021. In the month of December I made two payments to Cell C with regards to that account. Since the beginning of February 2022, I have received calls from a debt collector on behalf of Cell C claiming my account is in arrears. On multiple occasions I've contacted Cell C's call centre with the hope of trying to resolve this matter but no one is able to assist me. Today I got a call again from the debt collector saying I owe Cell C and that I've been labelled a bad payer! Someone from Cell C needs to explain what money do I owe Cell C and for what since that contract has been cancelled