
Following yesterday's iOS 10.3 release, several carriers around the world have enabled support for native Wi-Fi calling and VoLTE on iPhone.

In addition to updating to iOS 10.3, the features require installing a carrier settings update to be enabled. Your device should prompt you to install the carrier settings update in Settings > General > Software Update.
Features may be limited to certain iPhone models, so check with your carrier. British carrier O2, for example, has only enabled support on iPhone 7 and iPhone 7 Plus right now, with plans to add other devices soon.
The full list of carriers that have just enabled native Wi-Fi calling on iOS 10.3:Australia: Optus
Austria: A1 Telekom
Belgium: Telenet
Greece: Cosmote
Hong Kong: 1010
Norway: Telia
Poland: Orange, Play
Switzerland: Sunrise
United Kingdom: O2The full list of carriers that have just enabled VoLTE on iOS 10.3:Denmark: 3
Greece: Cosmote
Peru: Movistar
Poland: Orange
Russia: MegaFon
Turkey: Turk Telecom
United Kingdom: O2To enable Wi-Fi calling, open the Settings app, tap on General > Wi-Fi Calling, and toggle on Wi-Fi Calling on This iPhone.
To enable VoLTE, open the Settings app and tap on Mobile Data > Mobile Data Options > Enable 4G > Voice & Data.
Meanwhile, Verizon customers with an iPhone running iOS 10.3 can now make and receive Wi-Fi calls on other iCloud-connected devices, including the iPad, iPod touch, Apple Watch, and most 2012 or later Macs, even if the iPhone is turned off. The devices must be signed into the same Apple ID used on the iPhone.
Update: Slovenia's Telekom also officially announced VoLTE support today, although the carrier was already included on Apple's list.
Article Link: Wi-Fi Calling and VoLTE Expand to O2 and Other Carriers on iOS 10.3