I've had amazing success with the "Should I answer?" App.

https://play.google.com/store/apps/details?id=org.mistergrou...

>The database is built-up by the community of all users who have the application installed and/or visiting the application pages. The users can anonymously send ratings and reviews for public phone numbers (we do not collect info for private numbers) based on real answered calls. These ratings are stored in central server database where our admins can check them and after evaluation and approval the reviews with ranking are then distributed to all mobile phones with our application installed.