Please see page below about WiFi calling:
https://www.o2.co.uk/connectivity/wifi-and-4g-callingThe list of compatible devices from that page are:
iPhone Xs, iPhone Xs Max, iPhone X, iPhone Xr, iPhone 8 Plus, 8, 7 Plus, 7, 6s Plus, 6s, 6 Plus, 6, SE
Samsung Galaxy S9, S9 Plus, S8, S8 Plus, Note8, S7, S7 edge and J3 2017, A8, A3 2017, A6, J6, J5 (2017), A5 (2017), Note9, A7, J6+, J4+, A9
Sony Xperia XZ2, Xperia XZ3, XZ2 Compact, XZ Premium, Xperia XA2, Xperia XZ1, Xperia XZ1 Compact
Oneplus 5, 5T, 6, 6T
HTC U11
Doro 8035
There is another page that goes into detail about Android versions (for Samsung):
https://www.o2.co.uk/help/network-coverage-and-international/wifi-and-4g-calling#samsung-setupThis says that your firmware needs to include ‘TEF’, ‘O2U’ or ‘OTF’ in order to work...
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Link to our guide on how to contact them can be found here