CONTRACT CLOSED

I had a contract with MTN in 2014 and paid up the account and closed it and my number was converted to prepaid.Recently I had been receiving calls from MTN call centre 808 telling that that i am owing and should pay and when I phoned back it cannot go throught as my no is now a prepaid and I used my colleque phone so that I can phone with the contract no and the agent told me to ignore the no as it shows on her system as a closed account and she can't access my details.She also said she does not know who has been caling me.I was informed that my account was handed to al legal.I don't owe MTN please clear my name otherwise I will take the further steps.I understand taht MTN is struggling financilly at the moment bt it should not do this to the Clients.My colleque had almost same problem.