THIS is exactly why Google Voice is so powerful! There ARE some other reasons, but this is a big deal methinks!
Once you link your cell phone, home phone, and perhaps your work phone, you’ll be able to control which devices ring based on who is calling your Google number. You can assign certain callers to ring your cell or your work phone (or both), and can forward some calls straight to voicemail or directly to the spam folder.
via Google Voice Blog: Google Voice for Newbies: Forwarding phone numbers.