Google Voice is said to be absorbed into Google Hangouts in the same way that messaging on Android devices integrates. This falls inline with their vision of one app that merges the functionality of all social and communication features Google has to offer.
Once Google Voice is integrated into the Hangouts application you will be able to make phone calls through it in the same way you can on the web currently. These calls will of course be the same VoIP calls you are currently used to. Also there will be no need to have both Hangouts and Voice apps on your devices.
The merge of services is supposed to take place this summer and will then make Hangouts the one stop app for system wide SMS messaging, chat, and VoIP calling. We can assume that other services will follow soon after at the rate Google is going.
Google Hangouts is becoming one of the most feature rich apps on any Android device and is proving to be not only a good move but also a useful for end users. This could increase awareness of features like Google Vice that your average user may overlook.
We will keep you informed as summer comes closer as to what to expect and more specifically when to expect it.