*********ly handed over for no reason on a paid up acc

Had an account with Mellins, paid it up by swiping my card at one of their stores, suddenly I get a call from Barret legal and associates claiming that my account has been handed over. Mind you Mellins never phoned me not even once to inform me of an outstanding balance. How is it fair on my name that I have been handed over for an account I was under the impression I'm done paying and no communication was made what so ever from Mellins side. I sent Barret legal proof of payment that the acc was settled in store but they never even bothered to get back to me and inform whether they find it in order or not. How's that professionalism and fairness?