While not a lot of networks are already offering Voice over LTE services for their users, it is a technology that is said to be of great benefit for those who have strong data connections and who still make a lot of voice calls. Google’s Project Fi MVNO (mobile virtual network operator) announced that they are testing out this tech, more popularly known as VoLTE, to a select subset of users. They did not indicate which qualified you to be a part of this group, but this is still good news for those who have been selected.
Google said that one of the advantages of using VoLTE is that you get a higher quality of voice calls without your data allocation suffering for it. In case you also need to browse or use your data while you’re on a call, your connection will not suffer and you’ll be able to use your browser or other apps like you were not on a call. It also gives you a faster initial call setup experience.
If you’re a Project Fi user, you’ll know that your part of the subset that can test out VoLTE when your signal indicator will continue to display LTE when you’re making or receiving a call. Usually when you do that, you will see it falling to H (HSPA). Furthermore, if you have a Nexus or Pixel device, you might see a “Use LTE services to improve voice and other communications” in your Cellular network settings.
There is no way to force this testing or to apply to be part of it, so you just have to patiently wait and see if you will ever become part of the subset. Or you can just wait until they eventually roll it out for all Project Fi users.
Source: https://androidcommunity.com/project-fi-testing-out-voice-over-lte-for-some-users-20170220/