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if your phone is capable of wifi calling then you should be able to activate in settings in mobile. There is no setting for wifi calling in hub
What is your handset & was it originally tied to another network? When first launched BT WiFi Calling on Android was only officially supported on handsets supplied by them but it generally also worked on handsets bought SIM free. Handsets originally locked to another network with that network's version of Android required to be flashed with a "vanilla" version.
Google throws up a few threads were this is an issue with this phone but no resolutions. I did find this which suggests that the setting isn't in the calls settings but SIM settings. But it's a couple of years old & may not apply to your version of Android. If it's not there for you I can only suggest you contact the manufacturer.