Thank you for all your help. I finally managed to get VoLTE and VoWIFI working on my U11+ European DualSIM.
But it seems that the modifications described here are necessary (at least some of them) but they are definitely not sufficient,
at least for every European carrier. In my case they were not, for Orange Romania, the settings were enabled but the radio would not
register with the network for VoLTE and VoWIFI.
What I needed to do were the changes described for the OnePlus recent phones, with the PDC tool and the Qualcomm drivers.
I found in /vendor/mbn/mcfg/... the *.mbn specific files for many carriers, and mine Orange RO.
Took the mbn file from there and activated it in the PDC tool in usb diag mode (search for this in the OnePlus forums).
Then I just edited HTC__034.xml (my CID) for just these parameters:
" <item name="carrier_volte_available_bool" type="boolean">true</item>
<item name="carrier_wfc_ims_available_bool" type="boolean">true</item>
<item name="carrier_htc_volte_reg_icon_bool" type="boolean">true</item>"
, last one is optional if you want the VoLTE icon, but it's kinda big, only the first 2 are actual enabling functionality.
Then after reboot everything worked perfectly.
