Google said that it is testing VoLTE network connections for some Project Fi users. With VoLTE, phone calls use 4G LTE that results in better call quality and faster data connections. However, VoLTE will not use your monthly data to offer the service.
The company posted in Project Fi forum that it has started testing VoLTE with a subset of its users. VoLTE calls on Project Fi are most likely to work only when it is running on T-Mobile’s network. Google did not mention when all the Project Fi subscribers will get the VoLTE support.
Alena, Project Fi Community Manager posted in a forum,
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.
Introduced in 2015, Project Fi is Google’s own wireless service that was made open to all users in the US in March 2016. It works with Pixel, Pixel XL, and Nexus 5X and 6P.