iPhone 8 doesn't have a Wi-fi calling option in the phone settings

My wife's iPhone 8, iOS 16.7.10, doesn't have a Wifi calling option in the phone settings.

My iPhone 15 does have the Wifi calling option.

We are both using GiffGaff sims which use the O2 network.

We are both connected to the same home Wifi network.

Everything I've read tells me that Wi-Fi calling should be available but it isn't.

Does anyone have any idea why?

Dave

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