One app I've always wanted to write is one that intercepts incoming phone numbers that are not on your contact list and do a quick lookup on those reputation service sites and display it side-by-side with the number. Similar to how iOS has recently added the "Might Be John Doe" based on numbers it has learned about through iMessage or Mail.

Maybe iOS 10 has added enough functionality on the call screen APIs to allow it (I haven't honestly checked), but up until now there hasn't been a way to tie into calls like that without actually writing an entire VoIP backend so you could control the complete call flow.

Generally, if it's not in my contacts list, then it's unsolicited and flagged spam. I won't answer unknown phone numbers (as I get a large number of spam calls in Spanish for immigration and high interest loans coming from or spoofed to the same area code and CO as my number, so even "local" numbers are suspect to me). If I don't answer, they leave a message, and it is someone or someplace I know about, then I'll add it to my contacts. If I answer an unknown number, just the act of asking "Hello?" seems to be about the equivalent as clicking an unsubscribe link in a true spam message; it notifies the spammer that they've got a warm contact that they can resell or contact again.

There are websites for caller reputation. (If you can assume that an unknown number is suspicious).