My mobile provider - Three UK - offers WiFi calling, but only if you have a phone purchased directly from them. For everyone else, they have a crappy app which hasn't been updated in two years.
So, let's break out of their artificial restrictions and get some WiFi calling on the OnePlus 5T!
- Open your phone dialler app.
- You don't need to press send, you should automatically be taken to this engineering menu
- Touch the top menu item "oneplus Logkit"
- Scroll to the bottom
- Press "Function Switch"
- Select "VoWifi switch"
- Your phone will ask to reboot - so press OK to reboot
- Once the phone has restarted, go to your Settings menu
- Select "SIM & network"
- Scroll down and select "Wi-Fi calling"
- You can optionally switch the "Calling preference" to "Wi-Fi preferred". If you do that, calls will go via Wifi even if you have a mobile signal.
Put your phone into Aeroplane mode, then switch on WiFi. You should see this logo at the top of your screen.
Try to make a call.
If this doesn't work for you... sorry! You should speak to your network provider to see what they recommend.