Why does my phone lose signal indoors?

Poor indoor signal is usually not a network fault or a failing phone. It is the building, and the fix is to stop using the mobile network indoors at all.

Safety

Step by step

1. Turn on wifi calling.In the phone's settings under mobile or calls, depending on the make. Calls then go over your broadband and the mobile signal becomes irrelevant indoors. This is the answer for most households.
2. Check your network supports it.All the main UK networks do, though some smaller providers that use their networks do not. It is worth a two-minute check on their site if the setting is missing.
3. Understand what blocks signal.Foil-backed insulation and plasterboard, low-emissivity coated glazing, thick stone, concrete with steel reinforcement, and basements. Modern energy-efficient buildings are notably worse than old draughty ones.
4. Find where it does work.Walk round and note the signal in each room. The pattern usually points straight at the cause — one room fine and the rest poor means an internal barrier rather than distance from the mast.
5. Use the real signal figure, not the bars.Bars are unstandardised and near meaningless. On both iPhone and Android there is a field-test or status screen giving a dBm reading; above −85 is good, below −110 is unusable.
6. Toggle flight mode.On for ten seconds, then off. It forces the phone to reselect a mast rather than clinging to a distant one it latched onto elsewhere. It genuinely helps more often than it should.
7. Check the network mode setting.A phone locked to 5G-only or set to a preference that suits somewhere else may refuse a perfectly good 4G signal. Automatic is right for almost everyone.
8. Take the case off and test.Thick cases, particularly metal or magnetic wallet ones, measurably reduce reception. Worth a single comparison.
9. Check whether the SIM is seated.A partly seated physical SIM causes intermittent signal that looks exactly like poor coverage. Remove and refit it.
10. Test with another phone.If a second phone on the same network works fine in the same room, the problem is the handset rather than the building or the network.
11. Check the network's coverage map.Every provider publishes one, with an explicit indoor rating. If indoor coverage is rated poor at your postcode, no amount of setting changes will help and switching networks may be the answer.
12. Do not buy a signal booster.Most sold online are illegal to operate in the UK and interfere with the network. Wifi calling does the same job legally and costs nothing.
13. Consider a femtocell if wifi calling is not available.Some networks supply a small legal box that creates a mobile cell using your broadband. Ask the provider rather than buying anything third party.
14. Make sure the broadband is reliable.Wifi calling is only as good as the wifi. A weak signal at the far end of the house moves the problem rather than solving it.

Tips

• Wifi calling is the fix. Free, built in, legal, and it removes the problem rather than working around it.
• Ignore the bars. The dBm reading in the phone's status screen is the only meaningful measure.
• Signal boosters bought online are generally illegal to use in the UK. Do not.

Common mistakes

• Buying a signal booster or repeater online — Almost all are illegal to operate in the UK outside Ofcom's narrow exemptions, and they interfere with the network for neighbours as well.
• Judging signal by the bars — Bars are not standardised between phones or networks, so two phones side by side show different counts on identical signal.

If it doesn't work