Accusations of apparent 20yr old debt by Standard

I am inundated with calls that boarders on harassment from a debt collecting company DMS who tells me that they are contracted by Standard Bank to collect outstanding debt that is apparently almost 20 years old. <br> <br> Firstly, I have never received any communication regarding any outstanding debt from Standard Bank throughout the years.<br> Secondly, throughout the years my ITC credit profile has been clear and is still to date.<br> <br> I'm being harassed at my work place and I need this to stop ASAP.<br> <br> <br> <br>