I checked my Verizon Android one phone's call settings, and couldn't find a wifi option. I believe you're right, it's not available on this phone.
I used my Verizon-sold Moto E5 Play with WiFi Calling (and of course, VoLTE), but for some reason, WiFi Calling doesn't work on unlocked phones not sold by Verizon...like the X4.Interesting, thanks - now I know why I can browse the internet, etc., while in a call.