Turning on Wifi calling for Sony Xperia XZ Premium differs from O2's help page.
Settings:- Network & Internet:- Wifi:- Wifi preferences:- Advanced --Wifi Calling.
Hope this helps someone. 
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Yes it was discussed here.
https://community.o2.co.uk/t5/Other-Products-Services/Enable-VoWifi-VoLTE-on-my-new-Subscription-Son...
Unfortunately O2 haven't updated their procedure from nougat which had both settings in the "more" section of network, now volte is found in mobile network settings, and VoWIFI in WiFi advanced settings.
https://community.o2.co.uk/t5/Other-Products-Services/Enable-VoWifi-VoLTE-on-my-new-Subscription-Son...
Unfortunately O2 haven't updated their procedure from nougat which had both settings in the "more" section of network, now volte is found in mobile network settings, and VoWIFI in WiFi advanced settings.
