Google Voice

Google Voice is a free Internet service that uses VoIP technology to link phone numbers together. GrandCentral was relaunched as Google Voice on March 11, 2009 with new features, including voicemail transcriptions and SMS managing.
Users of Google Voice are able to select a single U.S. phone number, from various area codes. When a Google Number is called, any or all of the user’s phones may be set to ring. Which phone(s) ring can be set based on the calling number, based on contact group, and/or based on time of day. The service includes various additional features, like centralized voicemail and indexable, automated voicemail transcription, accessible by PC or phone.



参考：
http://www.crunchbase.com/product/google-voice
http://www.google.com/googlevoice/about.html