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 call to my cellphone on 04/28/17 @ 1:10PM. I did not answer call since nobody has my cellphone # that is not already on my contacts list. My call log only displayed the incoming phone #/Jacksonville, FL. Thank you to those that continue to post/educate everyone else of your experiences with unrecognized incoming calls.