There is no options to enable VoLTE.
Settings -> Network & internet -> Internet -> SIM1
If your cellphone provider doesn't support ZenFone 8 - you won't see such options (VoLTE and/or VoWiFi) in your phone. Put sim-card of different cell provider and you'll see such options there.
Or put your sim card to a different phone
Or put a different sim card in a different phone.
search in the settings for 5g
click a result from the "other" category in the search results! then u can find those hidden options!
it is a bug, that they do not show up in the data usage if u navigate there normally i hope they will repair it soon
this is a workaround to find it
Dial *#*#3642623344#*#*. Go to Settings -> Network & internet -> Internet -> SIM1. It should be there now. Of course, your network needs to be supported first though.