MTN harassing us for an account closed 2 years ago

I closed my mtn account 2 years ago at the end of its contract period with no outstanding fees or charges on the account.

Over the last month my mother (the next of kin for the contract) has been receiving countless messages and phone calls stating that I have an outstanding amount with mtn for an account closed 2 years ago.

There are a few flaws in MTNs contact centre: the caller will not identify themselves, the amount they say is outstanding changes with every phone call and sms as well as they have no contact details for me - when I had an account with them for 13 years.