Calls can't be made over 5G so a phone will always drop to 4G or 3G to make a call, however the signal indicator may continue to show 5G if the call is being made over 4G.
VoLTE is the name for 4G calling.
if you're always dropping to 3G and never 4G then VoLTE isn't working for some reason. Is it switched on in your phone settings?