Harassment for outstanding company tax but I am not connected. This is *****

I have received calls from Credit Solutions Services claiming to be calling on behalf of Sars wanting to talk to Francois Blaauw or the owner of Blaauw and associates. I do not know this person. CSS claim that they receive the file from Sars. They previously called my cell and I told CSS never to call me but today 21 December 2018 they called to my work office number. How do I resolve this matter?