Not sure, but I’ve never seen a setting for it with pie and I tried an unofficial Q build a while back; don’t remember seeing it with that either.
Lots of discussion on this topic recently, for example here and here.
Edit: I was confusing VoLTE with VoWiFi, did some research, and I’m still confused! 
How would you know if VoLTE was active on the phone? In the mobile settings mine is set to LTE preferred.In the status bar just the signal strength indicator is present.