The rumors about Google testing VoLTE first emerged when Project Fi customers started to notice the change. And true enough, they were part of the group Google chose to test the network connection on.
As its background, Project Fi can be considered as an MVNO that operates under Sprint, T-Mobile, and U.S. Cellular. Compared to other MVNOs, Project Fi doesn't rely on just one network. This is what makes it so different from other MVNOs since it can easily switch to the best network at any possible time. The service, however, is currently available on select devices such as the Google Pixel, Nexus 5X, and the Nexus 6P.
Out of the three networks it runs, T-Mobile is the only one that supports VoLTE connection. So if you're using Project Fi and you're in an area where T-Mobile's network is being used, you may be part of the people testing VoLTE.
And if you are, you may have already enjoyed some of the advantages VoLTE gives to Project Fi users such as a higher call quality, calls that don't count towards your data allocation, faster call setup, and faster data browsing while you're on a call. When you're using the network, you'll be able to navigate Google Maps while you're on a call and not worry about a lost connection.
Although Google has confirmed that they are already testing VoLTE on select Project Fi users, they have not identified a specific time when this service will be available to all users. The company promises that they would give an update as soon as they move to the next testing stage.