Thanks for posting! All BT SIM cards are enabled for standard 4G and 4G Calling (VoLTE) you will only get the latter if your device supports VoLTE and avalible on your local mast.
In regards to the BT Systems noticing the phone change, this is just a house keeping issue and doesn't effect the service you recieve. However if you want to help push the update to your account, you can by doing a few simple steps. If you still have the Sony handset put the sim in and send a standard text message to anyone (do not use any messaging service such as whats app), then with the SIM in the handset turn it off. Then ensure that the new LG is turned off, put the SIM card in and switch on and now send a standard text message. Over a the next day or so the BT System should update to show the new handset in the "My Handset/ SIM" section of your BT account.
On your BT Mobile Account it will mentioned Wifi Calling and 4G calling, these are for reference. Wifi & 4G calling (VoLTE) is only officially avalible on selected handsets. However if your device has the correct firmware it should work.