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.