Lack of communication

On the 16th of November 2021, I cancelled my contract with Cell C and it was terminated on the 16th of December. In January I received numerous calls from Cell C regarding payment that was due to them and I disputed the matter and I didn't receive any communication from Cell C. In April 2022, I got a final letter of demand which I disputed via email and yet again they did not respond. Recently I've received calls from debt collectors claiming I owe Cell C money and if I do not pay, I will be blacklisted.

I have indeed been blacklisted by Cell C but they fail to communicate with me what money is owed to them and for what reason do I owe them anything.