Google has finally begin migrating Google Voice to Hangouts by introducing free (and paid) voice calls on Android, iOS and the web. Google already had the calling feature available on their old Google talk interface but this feature took ages to appear in the new Hangouts apps and interface.
The new calling feature in Hangouts is being introduced via app update for Android, along with the release of a new dialer app. On iOS, the dialer app was already available, which is kind of odd that Google would introduce a new feature for a third party platform first. For Android, users would have to download the latest Hangouts update, version 2.3, which would be rolling out over the next few days. They would also have to download and install the new dialer app to enable voice calls. For iOS and web, users would just have the voice call feature enabled, the next time they open up Hangouts.
Calls to numbers in Canada and United States are free, while international calling is available for cheap rates, as listed here. Google voice calling on Android and iOS is a welcome addition and would save users a lot of money, specially on international calls.