This is not a question, but a description of an issue I recently had and it's solution - hopefully it helps others in a similar situation.
Issue: WiFi calling (and VoLTE / 4G calling) was set up and working fine, but suddenly stopped working in the afternoon on 8 September 2018.
Phone: Samsung Galaxy S7 (open market - not bought from o2, latest firmware updates installed)
Solution: Uninstall and Reinstall My o2 app.
Description: WiFi calling and VoLTE had been working as expected for a number of weeks now, but without warning stopped. It had previously been enabled via My o2 app.
I had made calls using WiFi in the morning of 8 September, but by evening it would not connect to WiFi calling.
Furthermore the WiFi calling option had disappeared from the settings menus. VoLTE was greyed out so wasn't allowed to turn it on.
My o2 showed I had both enabled on my account.
I disabled wifi calling and re-enabled it through my o2 (including waiting 24 hours and multiple restarts of the phone), but no luck.
So I started the phone in safe mode and WiFi calling was back! If I came out of safe mode the wifi calling disappeared again.
Looking at my app update history My o2 was updated on 8 September. I uninstalled and reinstalled My o2 and WiFi calling and VoLTE was working fine.
Either the update didn't install correctly for me, or there is a bug in the update process for the latest update, however the fix is pretty easy!
Hopefully this might save someone from losing all WiFi connection settings (as happened to me when o2 support asked me to reset all my network settings )
It's down for everyone at the moment it would seem.
Had same experience on Galaxy S8 supplied by O2.
Not for the first time on 17 Oct WiFi calling and 4G calling stopped working and disappeared from phone menus.
Entries are no longer visible via Settings-Connections or via Phone App-Settings (Three dots)
Reported issue again to O2 and they raised a ticket.
Previously got it back my disabling the service then re-enabling using MyO2. This can involve a 24 hour wait. Did this, but first time it did not bring it back. I tried this a second time and magically the menus and WiFi calling operation all OK again by morning of 18 Oct.
Don't know if my disabling and re-enabling the service was the fix or not.
WiFi calling for me is always a bit flaky. Despite a solid WiFi connection the WiFi calling feature can disable itself, although so long as menu entries are there I can re-enable on my phone.
Also when I reboot my phone, sometimes despite WiFi calling being selected on the phone it will not start the function. In this situation if I turn the WiFi Calling feature off and on maybe four or five times on my phone it will kick-in.
If only this was reliable and if only it could do text messages as well.
Really miss TuGo that did all that I needed and worked on any phone.
I hope O2 can get to grips with this. Whenever I report these issues they make out this situation is unique. Clearly others suffer the issues as well.