Telkom rodding customers

I have a massive issue with regards to telkom as I have paid of my cellphone contract years ago, and had no issues but then a few months ago I get a phone call and messages from lawyers demanding for payment towards a telkom account which was cancelled for about 3 years ago, and demanding payment of 36000 rand to be paid, and blacklisting me due to this. I am at a point of not knowing what to do and I have explained to them the issue and they have no sense of understanding anything