I have an android on Verizon. A few months ago, following a normal update, my phone started giving me a "Potential Spam" warning on some incoming calls. Heeding the warning, I have not answered any of those calls, and validating the warning, none of these calls has resulted in a voice mail. I figure that if it was a legitimate call they would leave a message, so this service seems to have value.

I’m grateful for it, not that I ever answer my phone if it’s not someone I know anyway.

Did you look up the number on Google or some complaint boards, like http://whycall.me? Sometimes if the numbers belong to scammers, people might have reported them online.