Calling in the US and Canada Free with Google Hangouts/Voice
Voice Calling in the United States and Canada added into Hangouts. The addition allows a group of people to also speak to another on the phone. It will be completely replacing Google Voice and has now added most of the previous functionality into the new Hangouts app. This keeps your number from Google Voice and doubles as a Google+ chat system, video calling, and now voice calling. Stay tuned and check out the new features with Google Hangouts. Google Voice is soon to be merged with Hangouts.
Check out the help page:
Read about Voice: