As spotted on Reddit, Microsoft seems to be trying out Xbox party chat on your phone. It was noticed in recent patch notes for the Xbox beta app.
This hasn't gone live in any official capacity; heck, it hasn't even been announced, but the the functionality was spotted in patch notes for the testing app. It will allow players to enter voice chat when they are away from the console, but more importantly, allow players to join parties in cross-platform games, which are becoming more frequent on Xbox One as it embraces PC.
All you need to do to try it out is download the beta app. As of right now, the beta app is only available on Android, but the full Xbox app is on iOS, so theoretically it will come there when and if this officially launches.
This sounds like nice, handy functionality, so hopefully it comes through the beta with no major issues.