Google finally announced their long rumored wireless carrier yesterday called Project Fi, and for the most part the service seems like a pretty good deal for most consumers in certain data usage tiers. If you missed part of the details on the service you can check out our comprehensive breakdown here. One of those details is that once Project Fi has been set up on a device, that it will allow you to place calls and messages from other devices like laptops, desktops, and tablets all while using Hangouts as long as your Google account is logged in. With that being the case, Hangouts would be needing to have to Project Fi support so that those who are invited to test the service could use the feature to call through Hangouts.
Google actually started rolling out the update to Hangouts yesterday following the announcement of Project Fi, but it's likely there are plenty of people who haven't received the update yet. In addition to this there will be plenty of people who this update won't matter for as it seems to be geared towards users who have a Nexus 6 and will presumably only be rolling out to those people, as the only changes are seemingly Project Fi related and the Nexus 6 is the only device compatible with the Project Fi service.
The changes that seem to be coming along with this update include references to a handful of different Project Fi functions, like call quality reporting just in case voice calls are less than desirable, and a possible prompt asking users if they want to make calls with WiFi or the cell network should they have WiFi on and connected. It's also possible though that Google may just make calls on WiFi the default if WiFi is on and connected to a network. There is also some reference to Google Voice with the new update, although it's unclear exactly how it will be incorporated. The new update to Hangouts also has references to how SMS and MMS messages are handled with Hangouts and Messenger on the Project Fi service, and it looks like Google is implementing the use of labels or indicators for Project Fi users so they know which number they're sending messages from when on WiFi. The update will be continuing to roll out over the next day or two most likely, but we have the apk here if you wish to grab it immediately. Keep in mind the changes noted here are Project Fi specific and probably won't matter if you don't have a Nexus 6.