Update: Google has officially announced Voice calls, text messages, and voicemail for the Hangouts Android app, rolling out in the next few days. Free voice calling between Hangouts users is also coming to the app for iOS, Android and the Web. Original story continues below.
Google Voice and Google Hangouts are continuing to merge, as some users can now receive voicemail and send Voice text messages straight from the Hangouts app.
The new feature was spotted by Android Police and Android Central, and appears to have reached some users on an opt-in basis. Once enabled, Hangouts will stop sending messages from your SIM card's phone number, and will start using your Google Voice number instead. The Google Voice app and website will stop receiving text messages entirely.
As Android Police points out, there's currently no way to choose which phone number you're using in Hangouts, so you shouldn't enable this feature unless you've completely switched to Google Voice for all your calling needs.
Furthermore, Android Central notes that Hangouts can't handle MMS messages from Voice just yet, and users will still have to use the Voice app and website to listen to text messages and set up call routing. The fact that Google hasn't made an official announcement about Hangouts integration suggests that the feature isn't quite ready for primetime.
Still, the migration is a good sign for Voice users, who had been left in limbo as Google built out its Hangouts service. While Voice users can currently place calls through the Hangouts app in Gmail, Google+ and Chrome, that feature hasn't been extended to the mobile version of Hangouts yet. Voice's deeper call routing and voicemail management features haven't made the leap to Hangouts at all. That'll all have to change of Google intends to get rid of the standalone Voice service entirely, as rumored.