Charged after a contract is cancelled and cancellation fee is paid

I cancelled my contract with MTN in January 2022 due to dissatisfactions in relation to how my sim swap process and migration of my contact number. I paid a cancellation fee and 6 months later I still get billed, I have contacted MTN a number of times to stop billing me with no luck. Why charge me a cancellation fee if you have no intention to cancel my contract ?