Three don't seem to have publicised this but native iOS wifi calling is now available.
If you go Settings > General > About that will force the phone to look for a Carrier Update. You should find one available. After installing (Three 27.1) you can turn wifi calling on under Settings > Phone.
If you go Settings > General > About that will force the phone to look for a Carrier Update. You should find one available. After installing (Three 27.1) you can turn wifi calling on under Settings > Phone.