Bad service

I had a contract with them 5 years ago and I paid it fully, last year in 2023 when I wanted to start a new contract with MTN they told me that my fully paid account is locked. I requested them to unlock and close my account no one could assist me up to date since 2023 it's now exactly a year since I have this one problem. I have been calling 135 and sending emails to their legal department but nothing helps.