Assuming you have 4G signal in your area.
Often it can take a short while for everything to change over fully.
O2 can usually send you settings for your phone to get the service you need.
WiFi calling can usually be done by getting the My O2 App.
Go into manage your account.
Go into device details
In there you will find WiFi and 4G calling
In there you will find out if your device is eligible and information on setting it up
You would then have to turn on WiFi calling option on your phones settings
