Are you crazy? First of all, simply don't answer the call if you don't know who's calling. Second, don't ever call them back. Either way, you are confirming you have a viable number and you will find yourself on many new lists. If they're legit, they will leave you a message.
Received a call from this number on 4/21/17 at 3:17pm. I picked up and heard a phone ringing, as if my picking up had initiated some kind of automated call-back. After a moment of silence, a man with a heavy Southern accent asked if he could speak to Karen (I am Karen). I asked who was calling and he launched into a speech. He said he was from some kind of organization seeking funds to support police and families of officers injured or killed in the line of duty. I told him to take my number off their list and hung up.