WiFi calling

[TE107]

I have the Xz2 on EE who have confirmed that it is a WiFi calling enabled device and my plan supports WiFi calling. When I go to activate it on my device however I cannot. The icon and text is greyed out on the status bar drop down. If I press and hold the icon it opens WiFi calling settings but even if I toggle WiFi calling on, it reverts straight back to off. Can anybody help? 

[pressefr]

https://ee.co.uk/help/help-new/getting-started-and-upgrading/setting-up-wifi-calling/setting-up-wifi...

https://support.sonymobile.com/us/xperiaxzpremium/kb/801930746ffcf563015c0bfaac370020da/#gref

Hope these could help.

A side step note: An alternative to Wifi calling is Google Voice + Google Hangouts Dialer apps. With an internet Google Number, you can call via Wifi and on any device besides your mobile phone; via website google.com/hangouts, within Gmail, Chrome Extention, via Google Hangouts Dialer App i.e. a tablet.