Telkom, Nudebt

I have a current contract with Telkom that I am paying every month, however a few weeks ago I got emails from a company called nu debt saying I owe Telkom a penalty fee of 800, outstanding debt of 429 and interest of 69.00. what unsets me is the fact that Telkom never contacted me for outstanding debt and I have a current contract with them but now I am being threatened with an adverse for debt I was not aware of. Why has Telkom never contacted me regarding this?