Well, we seem to go in streaks with the telemarketers, however, fortunately, we do have caller ID. Many times the number is unavailable, but also there are many times when the number is. If they don't have the number available, then try to listen carefully as to just what the name of the company they are representing is. Also, I suppose, if you see the number is unavailable, you could just not answer it and in case it is someone you know, they can leave a message.
The first thing you need to do is register on the do not call list, then after 30 days or so, if you receive an unsolicited call, you can go up and register a complaint by giving the name of the company or the phone number that called you. Your registration will not expire. Telephone numbers placed on the National Do Not Call Registry will remain on it permanently due to the Do-Not-Call Improvement Act of 2007, which became law in February 2008. Check out the site at:
https://www.donotcall.gov/default.aspx (https://www.donotcall.gov/default.aspx)