Nudebt - Old-debt that was never sorted out!

I paid up my account 1 March 2018, I was told by THEIR agents that my account is paid up and closed ON EMAIL, when i called Cell C recently to apply for a cellphone contract, I was told I still have an outstanding amount from 2015. I told them I had paid it up with Nudebt, I was asked to send them proof, I was called back by Cell C and was told that Nudebt never sent the letter to them. My credit score says I have been owing this account since 2015, with no update. This unprofessional and uncalled for!