From Wikipedia:

Google Voice is a telecommunications service by Google^[1] launched on March 11, 2009. The service provides a US phone number, chosen by the user from available numbers in selected area codes, free of charge to each user account. Inbound calls to this number are forwarded to other phone numbers of the subscriber. Outbound calls may be placed to domestic and international destinations by dialing the Google Voice number or from a web-based application.^[2] Inbound and outbound calls to US (including Alaska and Hawaii) and Canada are free of charge.^[3] International calls are billed according to a schedule posted on the Google Voice website.

The service is configured and maintained by the user in a web-based application, styled after Google's e-mail service, Gmail. Users must have an established US telephone service to activate Google Voice. Users must configure this and optionally, additional phone numbers that ring simultaneously when the Google Voice number receives a call. The user may answer and receive the call on any of the ringing phones. Google Voice provides additional features such as voicemail, call history, conference calling, call screening, blocking of unwanted calls, and voice transcription to text of voicemail messages.^[4] Received calls may be moved between configured telephones during a call.