Collecting for Paid up Account

I cancelled my contract with Vodacom (Autopage) more than three and a half years ago. When this was done I confirmed with consultant that account is paid in full, the 24 month term is completed and everything is up to date. Account was closed and I went to a new service provider. Now years later I'm getting mails saying there is an outstanding account and I owe money. How could an account be closed and the number ported if there was outstanding amounts on this account. I tried getting hold of Nimble for months now, NO response on email, even though their mails say I must respond to mails which I do, No phone calls, CANT get through to their offices or just get cut off. I'm completely in the dark and NO one is assisting or even trying to help. Now my credit score is being affected and I'm NOT HAPPY. This is the worst service I had in years from any company, and that for trying to resolve a wrongfully listed so called outstanding account that was settled and confirmed by the service provider.