Well this was a hell of a month for me.
I learned so much about this 2 options and now I finally got them to work !
I live in Israel, my cell supp is Partner (former Orange , but this will work for every carrier)
This is my guide, works on all Stable ROMs with NO root needed !!!
No pc, no root, no flashes.
Have a good one!
You have to ask the service from you carrier before !
They need to approve your SIM for that.
Just search "(your carrier name) WiFi calling"
Enable it and then do this trick.
1. disable the carrier check for this 2 options:
2. Now go to settting>sim and cellular>'your sim' (example: Partner IL) > scroll down and enable Volte and calles over wi-fi (or Wlan)
In the calles over wi-fi - choose WiFi preferred.
3. Go to Play store and Download this app: (like the picture) shortcut maker
Open the app
4. Select the "activities" tab
5. Scroll down and search "modem config"
Press on it.
6. Open the selected "MBNLoader" by pressing "try this shortcut"
7. Now you will see the toggle on the first one, ignore it and scroll down till you see: MBNLoader /vendor/................. (The path doesn't matter) /Vodafone/volte/Greece/mcfg_sw.mbn
Select it !
Now go back, Exit the app, And restart your phone !
Viola !!! Vowifi and volte are working !!!!
Pictures attached as you can see for yourself.
p.s: if this didnt work for your try choosing other carrier from that (huge) list. one of them will work like a chram.