I’m on Asda/Vodafone and my indoor signal has badly deteriorated over the last 6-9 months, so much that almost every call indoors drops at some point. If I go outside it’s better. I’ve had to change mobiles to one that has WiFi calling, that helps but there’s no way to set it to only use it so it keeps going back to the useless mobile signal and dropping calls.
I’ve also tried an O2 Sim with the same results. Both their coverage checkers say I should have no problems, however, using the Ofcom checker it says all four networks are “Limited” for indoors.
I’ve ordered an EE Sim from 1p for £1 to try, not that I’m expecting an improvement but you never know.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!