Microsoft introduced a new beta of the Xbox phone app today, which appears to include the testing of party chat on your mobile device. Presumably, this will allow you to chat with your Xbox party on your phone, similar to the way the Nintendo Switch works. While the Xbox app is available for both iOS and Android, this new beta appears to only be available at the Google Play Store.
The highlight here really is the ability to chat with your Xbox party when you're not at a console. There isn't anything official on this feature from Microsoft; we were first made aware of the update thanks to eagle-eyed Reddit users. Time will tell when this update will be available for iPhone owners as well.