All of the BT sims are provisioned for wifi calling, and whilst BT wouldn't ever make any promises most compatible handsets would work on wifi calling with a BT sim.
I've had first-hand experience though with 2 similar handsets (Samsung S7), where 1 was from an old EE contract and my partner's from an old 3 contract, and the one from 3 wouldn't ever work on wifi calling with the BT sim.
Samsung eventually putting it down to the operator putting there software on and somewhat locking down some handset features, which i was told can't really be altered, so for some people may just be a case of trial and error, and if needs be just use your cooling off period for either the sim or phone you get and return one if things don't work out.
As a side note, my MyBT account has never shown that i have wifi/4g calling enabled, both previously with the S7, and even now when i have a new iPhone 11 Pro from BT on contract, even though both features have always worked fine.
Hope this helps clear it up a bit.
Forgot to add, but also some android phones (mainly Huawei from experience) seem to still try to use 4g calling even when wifi calling is enabled. Usually turning off VoLTE/Wifi calling helps there too so may be worth trying on other handsets.
I got my Nokia 7 Plus (which is not on BTs' out of date list) from Argos sim free and Wi-Fi calling works fine both on incoming and outgoing calls.
I don't know about iphones but on Android just drag the notification area down during a call and it will tell you if it's a Wi-Fi call
I'll try the tip on dragging down the notification area - that will be handy for testing what is operating at any time.