MTN ZA exploting a client from an expired contract

I took out a 24 months contract with MTN! The contract expired in Feb 2023, I was paying R318 pm! March they billed me R1 145, I sent an email on 21 March asking why they are not canceling the contract and they ignored that email. April I was billed another R1 145, I sent an email to their customer care, that email was also ignored! 18 April I called them on this, and to my surprise, May I was billed R499, adding to the R2 300 that they say I owe for Mar and April! At this point they say I owe them R2 900, on an expired contract! I have been sending emails and these emails are not being responded to! Yesterday I called their legal department and the service consultant by the name of Lerato dropped the phone on my ears, when i was asking if my email of the 21 March is not considered communication to cancel the already expired contract!!! Now I am listed as a defaulter at credit bureau! How can I owe them after a contract expired, during my 24 months term of contract O have never missed a payment, yet after the contract expires and I ceased using the sim card in February, I am now a defaulter!