With some carriers support may be limited by device. O2 for instance is temporarily restricting compatibility to the iPhone 7 and 7 Plus, despite the two features being available on devices as old as the iPhone 5s and 5c on other networks.
Aside from O2, added carriers supporting Wi-Fi calling include Australia's Optus, Austria's A1 Telekom, Belgium's Telenet, Greece's Cosmote, Hong Kong's 1010, Norway's Telia, Switzerland's Sunrise, and Orange and Play in Poland.
New VoLTE backers include Denmark's 3, Greece's Cosmote, Peru's Movistar, Poland's Orange, Russia's MegaFon, and Turk Telecom.
Wi-Fi calling, enabled through the Phone menu in the iOS Settings app, may be particularly important to some iPhone owners. On its own it can improve call quality, but in some cases calls can also be redirected to Macs, iPads, iPods, and Apple Watches associated with the same Apple ID, even if the iPhone itself isn't nearby.
In fact iOS 10.3 also enables the latter feature for Verizon customers in the U.S. Verizon has had narrower Wi-Fi calling support since iOS 9.3.
The 10.3 update brought with it a number of other features, such as the Apple File System, Find My AirPods, and a new reviews system.