from wikipedia & google:
Google Voice (formerly GrandCentral) is a free Google-owned Internet service that uses voice over internet protocol (VoIP) to link customers’ phone numbers together.
[youtube=http://www.youtube.com/watch?v=sHIWUw6cf1U]
then watch this:
[youtube=http://www.youtube.com/watch?v=9IlvfEBJGx4&hd=1&feature=hd]
可惜，要住美國先有得用。