Pathetic service

I had a Cell C account which was handed over to DMC debt management and I paid it off in June 2023 and even have the paid up letter from them next thing I know a month later I pick up that Nimble listed the account again. I've contacted Nimble numerous times telephonically and via emails but they still haven't sorted anything out. I've provided them with this paid up letter numerous times and still haven't received any assistance or feedback on it. I'm beyond the level of irritation and frustration and extremely upset about this whole matter as it affects my credit score immensely. I need this to be sorted right now because I've been trying to get it sorted from July 2023 already to date so please!!!!!!!!