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.
Called, asked if "John" was there. I answered a terse "no". He said "no message, I'll call back later" and hung up. No John here. But I've gotten several calls asking for a generic name like John, Ann, or Mary on different phones. Usually when I say no, they go into a fundraising spiel saying they'll talk to me then. Phone makes a "bloop" sound when I pick up.