I have Virginmobile with a landline and 1gig BB. I have tried all the major mobile providers too. I have tried a Nokia bricks, Galaxy Ace, Galaxy S6, Galaxy S7, Galaxy A10, OnePlus Nord, OnePlus 6T, Pixel 6 with VirginMobile, O2, 3, Vodafone, Orange, Tesco etc etc.
Inside I can make and receive phone calls if I lie on my ground floor, with my ear to the floor.
Most providers say - good "outdoors only" which in my experience is exaggerating. We're lucky if we get 1G let alone 3G, 4G or 5G. The nearest antenna mast is only about 2 miles away however it is at the base of a granite hill. My house is the other side of that granite hill. Villages around me have similar issues. During lockdown my wife had numerous phone meetings whilst standing on the footpath 1/4 mile down the road!
It's a known village joke that we can't get a reliable mobile signal. The BB speed is good though! 🙂
EDIT : Oh yes. British Gas, nPower, Scottish Power keep paying visits as they can't connect to our "smart" electricity and gas meter via the mobile connection.
We also have VM VoIP landline. I have a disabled daughter so VM have fitted a battery backup that switches to mobile in the event of a power cut. That'll probably not work either in the event of an emergency!