The companies that sell your phone number to businesses get more money if they can verify you answered your phone. If you call them back, they can also sell your number for even more money because you followed through and contacted them. Plus, the longer you stay on the phone, whether to listen to a recorded message or to chew out the person calling you, the selling price of your phone number goes even higher and continues to increase according to how long you stayed on the phone. Companies are willing to pay good moneoy for phone numbers that respond to unsolicited phone calls. I worked in the industry for over ten years (thankfully for a legitimate company) and learned a lot of secrets of the trade.
That may have been true way back when. But all they actually have to do is plumb in a set of numbers into a dialer or computer and go for it now day's. Why purchase a list when its easier to just plumb those numbers in? Do consider maybe getting a call blocking device or phone.. You can google them and many are around $60.00 or so.