Why does my phone lose signal indoors?
Because the building is in the way. This is not a coverage problem in your area, and the usual fix is a setting rather than a purchase.
The one test that settles it
Step outside, a few metres clear of the building, and try again. If the signal comes back, the area has coverage and the building is attenuating it. If it does not, that is a different problem with a different answer.
This takes a minute and it is the difference between a fix that is free and a wild goose chase.
What blocks mobile signal
Foil-backed insulation. The single most effective blocker in a modern building. Metallised foil behind plasterboard or in the roof is close to a Faraday cage, and it is now standard in new build and in retrofits.
Coated glass. Low-emissivity double and triple glazing has a metal-oxide coating that reflects heat. It reflects radio very effectively too, which is why a room with big modern windows can have worse signal than one with small old ones.
Thick stone, brick and concrete. Older solid-wall buildings, particularly with steel reinforcement in the concrete.
Being underground or in the middle of a building. Basements, and rooms with no external wall.
Why the new house is worse than the old flat
This surprises people, and it is a genuine and consistent effect. The construction that makes a building thermally efficient — foil-backed insulation and coated glazing — is exactly the construction that stops radio.
The better a building is at keeping heat in, the better it tends to be at keeping mobile signal out. It is not a fault, and complaining to the network will not change it.
Higher frequencies are worse at getting in
Lower frequencies travel further and penetrate buildings better; higher ones carry more data but are stopped more easily. That is a property of physics, not of a particular network.
It is also why indoor signal can seem to have got worse as networks moved to newer technology on higher bands, and why a phone that shows 5G outside may hold a weak, slow connection just inside the door. See is 5G worth paying more for?
Wi-Fi calling: the actual answer
Wi-Fi calling routes your calls and texts over your broadband instead of the mast, using your normal number. Calls come in and go out as usual; the person on the other end cannot tell.
It is supported by all the UK networks and by essentially every phone sold in the last several years. It is a setting rather than a purchase, it costs nothing, and it solves the problem outright rather than improving it.
Two things worth knowing. Calls made this way generally come out of your normal allowance rather than being free on top. And emergency calls over Wi-Fi rely on an address you register with your network, because there is no mast to locate you from — worth setting correctly when you turn it on, and updating if you move.
What about a signal booster?
Be careful here. Cellular repeaters are regulated in the UK, and a device that rebroadcasts a licensed operator’s frequencies is not something you may simply buy and install: badly implemented ones interfere with the network for everybody nearby, which is why the rules exist. Only certain classes of device are permitted without an operator’s involvement, and the market sells plenty that are not.
Given Wi-Fi calling exists, is free, and solves most of the cases people buy boosters for, it is worth exhausting that first. If you genuinely need one, ask your network — some offer their own approved units.
If the signal is bad outdoors too
Then it is not the building, and the questions become different: is it everywhere or one place, is anyone else on your network affected, does another SIM work in your phone. The mobile signal troubleshooter works through them in the order that rules out the most.