Do you happen to have a "EE" branded SM-G975F (S10+)?
I have a SM-G975F Flashed to EE's (EVR) firmware from sammobile and I'm on AT&T in the United States. This method worked for my S9 to gain VoLTE, but I don't see WiFi Calling or a toggle for VoLTE in the connections settings on any current roms available (BTU, XEU, VOD, EVR ETC.).
Does VoLTE / Wifi calling just "work and enabled out of the box" for you or did you need to toggle it on? Does the place to toggle it on / off even exist on the phone (I'm hearing mixed reports on that as well).
Nope, just direct from Samsung with standard unlocked firmware.
Was all enabled by default although there's a wifi calling option in the Quick Panel. Nothing for VoLTE but again on by default.