Contract fone

I had a contract with mtn i took out a samsung a13 i realized that i was suppose to have finished my contract in february 2023 but i cintinued getting billed when i went to the store and enquire i was told to call mtn call centre and cancel my contract which i did but to date they are still sending me a bill threatening to blacklist me if i dont pay please can you assist me on this matter I D number ***