@communicator1965 currently O2 WIFI calling is only available to contract customers although they can be sim only contracts or contracts that also include a phone.it is not available to PAYG customers.
One further fly in the ointment with O2 WIFI calling at the moment is that it only works with Android devices that are running O2 firmware (operating system version) so will only work with Android phones bought from O2.As far as I can establish it will work with any iPhone that is capable of WIFI calling though.
I agree that this would be a great addition to giffgaff's services but I think there's a long way to go before it will be available.
Details of O2's service here (FAQs at bottom of page):-
Thank you. The O2 site is handsome but not the easiest to understand.
I followed your link and just for interest checked whether I could upload Wifi Calling on my iPhone 6s under the latest OS11.2.6 . Coincidentally, I just received a newsletter from the " IanswerGuy " who offers advice on Apple kit. They both refer to a feature button called WiFi Calling that needs to be enabled. (Settings/Phone/Wifi-enabled)
That feature button is not listed on my phone. Does that mean the service provider has to download firmware to enable it to work, or is there a problem with my phone?
@communicator1965 what I know about Apple products you can write on the back of a postage stamp in capital letters but I would guess that the WIFI calling settings would be included in the carrier profile that you load onto the phone when you first join the network to give you the required internet APN settings etc.