Falsely handed over with no response

I cancelled my mtn contract and paid all the outstanding amount plus the early termination fee. I got an e-mail from Nudebt that MTN handed me over after I cancelled my contract. I contacted MTN customer services, MTNlegal apartment and Nudebt with no responses. How can a company hand you over falsely and no one can rectify the problem? Nudebt just keeps on sending demand but does not respond to e-mail.