Account issues

I had an account with Cell c which was transferred to this company after I lost my job, I have been paying the amount of 150 from 2023 from November 2023 to be R500, i paid up full my account, and requested a paid up letter and on the letter it was saying I am still owing R900. I spoke with the lady by the name of Thulisile she said that there are missing payments on June 2024 I didn't pay o which I paid, she asked for my bank statement to confirm. I sent the bank statements to her and she said she is escalating the matter to her supervisor, I have been waiting for more than two weeks now. I paid my debt full, I am so disappointed and frustrated.