Mtn *****

My contract term expired on 22 December 2021. I contacted MTN a month before the expiry/end date as well as on 23 December 2021 and requested that my number be moved to prepaid. I made it clear that I don't want an upgrade nor any contract with MTN. I received a bill during January 2022 in which Mtn charge me an early termination fee. How can I be charged a termination fee if the contract came to an end?