I am not happy at all

I don't even know where to start with MTN. They handed my account with credit for an account that was closed in 2018 already and they are demanding me to pay the outstanding balance. I don't owe Mtn any amount I paid the closure fee on my 25tb debit order installment. The way I was transferred from pillar to post when I called the call center for a paid up letter I still haven't got it. A month later