If you don't recognize the number, don't answer the phone.
Like others have said, register your number with the national Do Not Call registry at
https://www.donotcall.gov/default.aspx
It's free -- there's no fee to register, and it's good for land lines & mobile phones.
If you have any small children in the house, just hand the phone to them & tell them to talk to the nice salesman.
Cerek posted a story (or link) about a comedian who plays with these callers. He got a call for carpet cleaning & asked the guy if he could get blood stains out of carpet, furniture, and curtains.
Or, as soon as you hear the caller start a speech, just hang up...