Project Fi is Google's own MVNO which is unlike most others out there. But it is missing a few key features that most other carriers have at this point. Which includes VoLTE support. However that might be changing in the near future. According to a post on the Project Fi product forum, Google is currently testing VoLTE support with a handful of subscribers. This is something that Google routinely does with their other services, so it's not surprising to see it happen with Project Fi.
VoLTE is a pretty important feature for Project Fi, especially with carriers moving away from traditional voice networks and to doing calls over a data network like LTE. But since this is using phone numbers and phone calls, this testing is likely going to take a bit before it rolls out to everyone. For most people, they won't notice a difference between a VoLTE phone call and a traditional phone call, although VoLTE typically has HD Voice included, which means that people will get even better sounding voices when calling. Google hasn't said just yet when all subscribers will have this functionality available to them, but expect an announcement from Google when it is. Right now they are just looking to iron out all of the kinks before allowing everyone the ability to use it.
For those that unaware of what Project Fi is, it's a MVNO that uses three of the five largest US networks. That's T-Mobile, Sprint and US Cellular. The service will also automatically switch between the three networks and WiFi based on who has the best coverage at the time. It also connects to public hotspots to help keep your data usage down, and it'll automatically set up a VPN so that your data is safe when connected to public networks, like those found at Starbucks, Burger King and others. Currently, Project Fi only works on the Nexus 6, Nexus 5X, Nexus 6P, Pixel and Pixel XL, and that is largely due to how the smartphone switches from network to network, but Google is expected to open it up a bit more later on.