CELL C ACCOUNT

I took a data contract which I have cancelled in March 2021. Cell c continued sending statements and debiting money from my account even after I have cancelled my contract with them. I followed up with them and finally got confirmation via email that my account is been closed. Two months later after the confirmation Cell c started sending me statements and receiving calls from debt collectors. I recently received a letter of demand and I'm not sure with regards to what as I have proof that my account has been closed in September 2021.