Over the past couple of months I've been using wifi calling on my iPhone with Three. I recently had a replacement iPhone and realised I'd not shared how to actually enable this.
- Tap Settings
- Scroll down and tap on Phone
- Tap Wi-Fi Calling
- Tap the toggle on Wi-Fi Calling on This iPhone
- Tap Enable
You should now see Three WiFi Call in the status bar on your iPhone (top left).