WiFi calling - how does it work?

[Frogger]

I have been all over the O2 site and cannot find the answers.
When a call is set up on WiFi Indios and I move out of WiFi area O2 say the call seamlessly moves to 4g.
Is this via the data capabilities of 4g or using Voice over 4g (VOLTE) which is no in my area yet. ?
Will the call transfer to normal telephony on 3G / 2G ?
In other words will the call drop when I am engaged in a call and there is no Voice over 4g locally?

I fell O2 have been economical with the facts here?

[MI5]

Yeah, be interested to know how a data only network can carry voice calls too.

[MI5]

Even Apple say you have to be in a VoLTE area https://support.apple.com/en-us/HT203078

[Frogger]

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@MI5 wrote:
Yeah, be interested to know how a data only network can carry voice calls too.

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I only say what I actually found to be true.

Skype, FaceTime, BT Smart talk and I believe WhatsApp are methods of carrying voice call over a data only network using a data only network (using Voice over IP )

[MI5]

Not disputing what you say - I'm just trying to find the logic.

[Frogger]

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@MI5 wrote:
Even Apple say you have to be in a VoLTE area https://support.apple.com/en-us/HT203078

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Quite correct it says to ‘make’ a Voice call over VoLTE network you have to be in a VoLTE coverage area. This is true, I cannot ‘make’ a call, but I can continue the call using Voice over IP using 4g data providing the IP connection. 

Have you a suitable iPhone on O2 which you could try for yourself? I have set up a call when the phone shows O2 WiFi Call, then walked away from the WiFi and there was a momentary break in transmission and the phone showed as being on 4g network and the call continued, if I then go out of 4g coverage it won’t hand over to 3G, it disconnects. When away from WiFi I cannot setup a call on 4g as it reverts to 3G when I dial. This is all of course assuming the wilds of the Norfolk Broads do not have VoLTE as is currently the state indicated on the O2 coverage site. 

Its all magic to me but, I only say what I see when it tested it. I have also noted it does not seem to hand back to WiFi when I return. 

Apple also says you can send sms text message using WiFi calling... you can’t with o2 (yet)

[MI5]

I can't test on O2 unfortunately as my iPhone is locked to EE but I've never tried it on that either.