This article inspired me to install Hiya on my iPhone to block spam calls. I will find out eventually how well it works. I get 5-10 robocalls per week, mostly from the same exchange as my mobile number or my Google Voice number.
It seems to rely on a central service for all blocking rules, including an optional whitelist for your contacts and an optional rule based on your phone number for “neighbor” spam. It can identify and block a gigantic list of known spammers and robocallers, which is beyond what I need.
Having looked at it, and its competition, it seems like there isn’t a good on-device-only call blocker for the super-privacy conscious who, like me, only want to whitelist contacts and blacklist all other numbers in a couple of area codes and exchanges. I really want to write one. It would have to be free, I think, based on the free competition out there—but it may fit a particular niche that is out there.