Google added global spam filtering feature in Google Voice which able
to automatically redirect calls, texts, and voicemails from any of the
numbers in our database directly into your spam folder just like Gmail
filters spam before gets into your inbox. After collecting enough spam
calls marked by Google Voice users previously and now they are able to
judge which numbers are spam calls and then dump this calls from into
Spam folder.
You can enable Global Spam Filtering feature on Calls tab of Google
Voice settings by checking box next to Global SPAM filtering. Spam
call library will keep growing as user can unblock calls with “Not
Spam” button in your spam folder if a number ends up incorrectly
marked as spam.
via pupuweb Blog [
http://www.pupuweb.com/blog/global-spam-filtering-
google-voice-block-unwanted-calls/]