Harrasment calls

I have been contacted several times from this company claiming to be calling on behalf of Sars. They have my cellphone number and my work telephone number. They ask to speak to Francois Blaauw or the owner of Blaauw Associates. I do not know this person but I keep receiving calls from them to discuss outstanding tax. How do they receive my contact information? If they are are receiving the information from Sars who is authorizing Sars to divulge my contact information and why are my details listed for this Francois Blauuw. Your urgent assistance is required.