Hi
the only way to stop 99% of unwanted sales calls is to have your phone line set so it will not accept anonymous calls, Yes that means a few of your customers cant ring on a landline, but they most people have a mobile too !!
I did exactly this about 3 years ago & never for a moment regreted it, I can actually do more work now as I am not on the phone to idiots all day trying to sell me something I have never heard of.