Nudebt not doing there job

I previously had a contract with cell c , last year July 2016 , the contract was cancelled and handed over to nudebt management for collections , I paid the full amount that was outstanding and confirmed with Cell C that the account is closed. But this month nudebt management has been contacting me to no end to claim there is an amount outstanding , after numerous emails with the proof of payments attached , I am nowhere with Nudebt management , they still claim that I am owing money?I get no help from Cell C to resolve this.