BT activates the sim for Wi-fi calling and 4G calling at the same time.
If you are using an iPhone, check the following setting
Mobile data- data options- enable 4G and select voice and data.
On the iPhone it just shows 4G as normal.
Result. I phoned BT abouttwo days ago to ask about Wi-fi calling. they said they would call me back after looking into why it was not working. a chap has just this minute called and said they have activated it on my account. I switche the phone off and on and there it is
Presumably this will be just activated at some point even though the BT system doesnt seem to know what a Nokia 8 is.
BT is migrating all the sim for the new services in batches. So at some point yes your sim will be updated.
As you pointed out BT do not support Nokia handsets but some users are able to get handsets currently not supported to work even if they are not officially supported proving the handset is able to to provide the service.
The updates are to the sim and not the handset.
Nokia phones are not officially supported by BT, currently its iPhones from 5s onwards for Wi-fi calling, 6 onwards for 4G calling. And for Android just the Samsung S9/S9+
Some other android handsets may work but won’t offer any support for these devices.