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 me this morning asking for "Rachel" who happens to be my sister in which my phone number is registered under. I asked who was asking and he hung up the phone. Called back and got a busy line. Going to block this #.