When I see a call from an unknown number on my caller ID, I answer the phone with a made-up company name. Unless it's a recorded robo-call, they will hang up. Example:

"Good (morning, afternoon ), thank you for calling 'Universal Motors', how may I direct your call?"

Calls have been on the decline.

Another note: They are now using cell phones to call you; probably no-contract phones. These are impossible to trace, so the Feds have hit a dead end with the 'Do Not Call' law. You can not call them back, because they either do not set up their voicemail, or you get a recording asking you to enter your phone number for their 'Do Not Call' list. If you enter your number, as it has already been pointed out, you are merely letting them know there's an active phone number.

I called this number back and received this message:
"You have been contacted for a marketing research or sales call"

A-hole company calls sometimes 3-4 times a day.  No one on line when you answer, after a pause the line is picked up and then hung up.  Calling the number results in a BS recorded message saying you are being contacted for "research" , "or a sales call".  No one answers the phone and the line hangs up after the recorded message.  They need to do some jail time!!

As you can see these [***] have been running free for quite some time...