I had a contract phone with them, for almost 2 years and decided to cancel the contract last year October 2023 and they charged me cancellation fee, and changed my number to prepaid

I had a contract phone with them, for almost 2 years and decided to cancel the contract last year October 2023 and they charged me cancellation fee, and changed my number to prepaid. Now I have discovered that my name was listed into bad payers, now my credit score is down because of them, when we still had contract they used to send me a statement through email that shows the balance, now I just saw the balance when I check my score, they haven't even bother to contact me or send an email to say you owe us. I'm very disappointed