MTN DOUBLE CHARGING CONTRACTS

I had a contract with MTN. I paid it up in August 2024. After closing the account, I requested a paid up letter from MTN which I have received. As of April 2025 I have been receiving calls, messages and emails from NuDebt in regards to my MTN account. After various calls and emails I cant get any clarity from MTN or NUDEBT why this account hasnt been closed. I would advise any one considering opening a cellphone account to steer clear from MTN