Today, GrandCentral announced a new community-wide service designed to fight annoying telemarketers and other phone spam agents. The PhoneSpam filter lets both visitors and GrandCentral users report unwanted callers to a community list at http://www.grandcentral.com/stopphonespam. Once the number is confirmed, it is added to the PhoneSpam filter. For GrandCentral users who have the spam filter enabled, calls from any of these numbers are automatically sent to the spam voicemail folder.
To avoid malicious abuse, I’m assume that “confirming” the number means GrandCentral does a manual check to make sure the number is a legitimately annoying one.