I've posted this before, but here's another suggestion for those who are especially bothered by these calls, at least if you have caller ID on your home phone.
About a year ago, I bought a new phone system for the house. It has a base unit with answering machine, and several remote wireless units. It's a very good phone system in its own right, but it has a feature that no other system has, to my knowledge.
This one lets you put a block on up to 20 specific numbers,
in addition to any number that has caller ID blocked (like the ones that just say "unknown number" in the caller ID window). There are a couple of other systems that do the same thing, but this system is unique.
Here's the thing that makes this system unique: Since caller ID information is transmitted between the first and second rings, other phone systems will ring once, then block further rings and disconnect. I didn't want even that initial ring. This system
always suppresses the first ring, so you're not bothered at all. If you hear it ring, there's a reasonable chance you actually will want to answer.
Whenever I get a questionable call, I simply block that number. When I get up to the limit of 20 blocks, I just delete the oldest few blocks and keep adding new ones as needed. I maintain a list of blocked numbers and when I blocked them, and find that when I need to delete the oldest ones, they are always more than six months old. Those generic "unknown number" calls don't count against your 20 blocks.
If you're interested, it's the Uniden DECT 6.0 system, model 2188-3 (there are various systems with different numbers of remote wireless units so you can buy the number you need), and the whole thing cost me less than $100. Amazon and others carry it. Well worth it in avoided aggravation, and I highly recommend it.
Amazon.com: Uniden DECT 6.0 Corded/Cordless Digital Answering System with Three Cordless Handsets (DECT2188-3): Electronics