Is there a device which will increase the strength of a BT mobile signal inside a house in a region of poor mobile signal?
Is your phone not capable of wi-fi calling?
If not, I believe there is a booster available from BT
You may find this useful: https://home.bt.com/tech-gadgets/phones-tablets/how-to-boost-your-mobile-phone-signal-11364261944709
Some networks have Femtocells available which are small mobile receivers that connect to your broadband and effectively create a localised mobile network. I'm not sure whether BT/EE do but I'd suggest looking into that avenue...
I've just found this on EE's website... https://ee.co.uk/help/help-new/general/signal-box/improve-your-signal-at-home-using-signal-box