It's been a long loooong time coming, but Google Voice is finally getting more-or-less fully integrated into Hangouts. Earlier today we saw Google migrating Voice messaging into Hangouts, and now with an update to the Hangouts app for both Android (version 2.3, which will be rolling out over the next few days) and iOS you'll be able to make phone calls with ease.
With Google Voice in Hangouts you'll be able to call other Hangouts users for free, place free calls to phone numbers in the United States and Canada, as well as take advantage of low international rates.
Here's how it will work: When you call from Hangouts, it will now use your Google Voice number, and calls to your number will come into Hangouts as well. The Hangouts app will also send SMS text messages via your Google Voice number. Voicemails will appear in Hangouts chats as part of the conversation with both transcription and audio playback. Google hasn't flipped the switch just yet on these features, but will be doing so in a few days.