Contract Cancelled after I paid all my arrears, But I am still receiving billing invoices

I still don't understand why I am I receive SMS from cell c legal company after they have cancelled my contract and give sim card to some costumers, But I am still receiving billing invoices from their legal company, I don't get clarification of what I am owing is been 2 years try to solve these, but unfortunately I cannot reach Cell C legal department because they are the one having these case, I was told i have to use cell c number to be able to transferred to their legal department but unfortunately my number was taken immediately after finishing paying my arrears.