It is strange and ad hoc. I have two S7’s (Mine and my wife’s). My wife’s has BT Wi-fi calling and 4G but is connected to EE automatically, mine connects to BT, doesn’t give me the option of connecting to EE but cannot get BT Wi-fi or 4G.
Both mobiles bought at the same time from the BT Shop.
You can now purchase a signal assist box that plugs into your BT broadband hub. This will create a 3G signal indoors upto about 15 meters and cost about £20 avalible on the BT shop.
BT only guarantee handsets with stock firmware on S9/S9+ and A8(2018). Other will depend on the stock variation and updates from android.
So you spend several hundred pounds on a new mobile from the BT Shop and then have to buy another device from the BT Shop to use all the functionality that the mobile has!?
You could look at it that way, or you could say that BT never said Wi-Fi or 4G calling would be avalible on all handsets. In regards to having poor signal thats not unique to BT Mobile, and thats why one of the reasons they offer a cooling off period for you try and see if the mobile works for you.
I have the same problem on S7 Edge. BT advertise that 4G is available on all sim only packages but it is not true it seems to depend on which handset you have.