Hello Pablo, I had a go at opening the unisoc engineering mode to do the confirmation test, but nothing in the telephony menu looks like it selects LTE only. There are a lot labled VoLTE but they appear to be for inputting and applying settings.
I used the code: * # * #83781# * # * for spreadtrum / unisoc / ZTE. (No spaces though. Something in the posting pane didn’t recognise the * symbol unless it was spaced.).
Hope this helps to confirm VoLTE for some courageous z40 owners. I’m still scared of engineering mode…