Debt collectors

After being a loyal cell c client for many years i decided to terminate both my contracts due to financial circumstances after covid and lockdown. I cancelled both contracts and they are now prepaid. Months after i get harrassed by one mdb and now shapiro lawyers demanding payment. Both contracts had to be paid up in full before i could terminate them. That said, for months cell c is still attempting to debit R45 from my fnb account, nobody i speak to seems able or willing to find out what this debit order is for or to cancel it. So cell c is taking money from me and same time putting debt collectors on me. What is the problem Cell C??