Gave nimble group my 10k for nothing!!!

I had a Cell phone contract with Cell c which got into arrears after I lost my job, when I was reinstated at work I phoned them to ask how much am owing, they said around 10k, and the debt has been handed over to nimble group, I then contacted nimble group and made arrangements to pay off the debt which I did, it's been 2months since I paid it off but am still listed for the 10k which appears on my credit checks with defaults ..... When I call cell c they say the account has been handed over to nimble so they can't clear my name, while nimble is saying I was listed by cell c so they can't clear my name, meaning I just threw my 10k on the river, I regret entering into a contract with cell c