For better call quality and usability improvements, Google has been testing VoLTE with a subset of Project Fi users. VoLTE allows users to stay connected to the 4G network for browsing while making and receiving calls. It does not consume any data from your data pack.
Google recently announced that it’s been testing VoLTE service in LTE network over some of the Project Fi subscribers. The Google MVNO is currently only testing VoLTE on T-Mobile’s network. So, if a Fi subscriber is in T-Mobile’s network coverage area then he can test the feature. Google posts that VoLTE will not use any of your data allocation.
According to Community Manager Alena - "You can tell you're making a VoLTE call because your signal indicator will continue to display LTE instead of falling to H [HSPA] when you make or receive a call."
Google Project Fi was launched in the year 2015. Its only support Pixel & Pixel XL, and the Nexus 5X & 6P. The service cost of the Google Project Fi is $20/ month.