I own a small business and get quite a few. If I don't know you then you best leave a message. If they don't leave a message, I google the number. Most of them show up in "1-800 notes.com" so you can at least find out who is calling.
Remember these people, agencies and companies share information. They pay a lot for a list of verified, active phone numbers (where someone called previously and the owner answered the call). So by even answering the call, you almost make it worse down the road as they now know for sure that your phone number is active and it takes calls.
Most smart phone have call blocking ability build in, learn to use it. I have hundreds of numbers blocked. I still get a few calls but it's not nearly as bad as it used to be.