Never ever acknowledge them (or cold call telemarketers) in any way whatsoever if you hadn't picked up the telephone. This is feeding right into their hands. Scammers have little idea if their messages were ever received. By calling back it is confirming the call was received. More of the unscrupulous ones might even set up their devices to hound even more and/or sell your number to someone else. You could end up getting many more calls then ever imagined. The best way to fight back is through utilization of Caller ID. Knowing it can be easily be spoofed it's best to let it go through to an answering machine if you don't recognize a number.
Same goes with emails. Very early on I was dumb enough to click on that little button at the bottom that asks if you wanna be removed from the mailing list. DUH... by clicking on that it confirms your email address is a valid one which otherwise they'd have no way of knowing. Their automated equipment then compiles a list of all the valid email responses which they can use and sell to others.
As far as pests coming to the door, I have front & back cameras. I always first check on who's there never having to deal with them.
Here's one more ace I have up my sleeve. Whenever I receive an unrecognized caller when sitting by my computer I click on a shortcut I've created to this audio file. Then hold the telephone to my computer's speakers.
How This Works
Robo calling systems recognize 'Special Information Tones'. The set you hear above tells a robotic system to delete a number from it's database because it thinks it's invalid. If you will notice whenever you first pick up a phone and it's a robo call there is a certain delay period before it begins. That's because the robo machine is listening to determine whether to proceed if a human answers. Hang up if it's a voice mail tone OR eliminate future calls because of these tones and the speech recognition patterns which follow them. I can tell you from experience, it does work.
Another thing I noticed. Sometimes I'd go weeks without robo calls. Then suddenly two or three would start appearing daily. I can only assume it was from the same robo machine no matter what the Caller ID said. Sometimes within minutes of each other. A day or two playing that audio file, they'd disappear sometimes for weeks.
Course this didn't stop those damn live telemarketers calling from Philadelphia for years wanting to clean my chimney. I finally decided to answer each time and ask what the hell they're talking about. "I don't have a chimney!". Course I really do but it does look like they've given up for now. It's been about three months. Fall is coming again, we'll see.
Graphic Courtesy Consumer Reports "Rage Against Robocalls"