Indoor mobile signal: best networks for coverage in buildings

Why your signal might be poor while indoors

The best way to explain why your phone signal suffers when you’re indoors is to think of it like going through a tunnel in your car and losing your radio signal. The two work very similarly and can be blocked out by solid material.

When you’re indoors, the phone signal not only has to reach you but has to do so through inches of brick and plaster. It works best when there’s no disruptions at all, but even then it can bounce around a bit.

Much like sound, phone signals can be broadcast on low, medium or high frequencies. Think about how you can hear the bass from loud music through walls better than the vocals, same goes for low frequencies phone signal.

Generally, the lower the frequency, the better the signal will be at getting through building materials, as we explain in the next section.

Some networks have better frequencies for getting indoors

Only four networks in the UK actually broadcast signal (EE, Vodafone, O2 and Three). The rest of the providers use those signals for their network.

Think about radio stations. They can’t all broadcast on the same frequency otherwise they’d clash. So, each network uses a different signal frequency depending on the mast, which affects indoor coverage.

There are multiple types of phone signal. There's GSM, 2G and 3G which are your conventional phone signals. Then you have 4G and 5G for data signal, though 3G can be used for that too.

The way it works out is that some networks will have better phone signal for getting into buildings, where others will have better data signal for getting into buildings.

Why there’s no one network better than the others for indoor signal

As we said before, the best frequencies for getting through walls are lower ones. Comparing the networks against each other, O2 and Vodafone broadcast their 2G/3G call signal on a low 900 MHz frequency against EE and Three who broadcast theirs at 1800 MHz and 2100 MHz.

So you could say that Vodafone and O2’s phone signal is better for getting indoors. But it’s not a slam dunk. Imagine a TV is turned up loud and closer to you. You’ll be able to hear all the frequencies of sound, not just the low ones.

So if you’re closer to the mast of one network than another, you’re likely to get better indoor signal on that one regardless of frequency. It means EE and Three aren’t necessarily a write-off either.

And now indoor signal is less of an issue anyway, as all major networks provide WiFi calling and 4G calling to give you more ways to stay connected (though not every provider on these networks gives you these).

Useful link: Solutions to poor indoor mobile signal

You can get a test SIM before committing to a network

If this is a big issue for you, it’s possible to test which network will give you the best indoor signal in real life. You can do this for free by ordering a pay as you go SIM from them.

Each of the main networks offers free pay as you go SIMs that you can order on their websites (see links below). If your phone is unlocked you can test each SIM in it one after another.

This is totally free to do. You don’t need to top up, as we’ve tested and every network lets you call their top up line without credit, or you can just have someone call you on the provided number if the SIM is active.

Turn off 4G calling and WiFi calling and listen to how well the call works. It can be hard to tell between them, but so long as the call comes through clearly you know you’re safe to join that network on pay monthly.

You also test a network during your returns period

The free SIM method can be a bit of a fuss for most people, but there’s another way of checking by ordering a pay monthly SIM even without testing it beforehand.

By law, each British consumer has 14 days to change their mind after buying their SIM online. If you want to try a different network, the one you bought from is obliged to let you leave. BT even give you 30 days to test their SIM.

This means you can order your SIM to test if everything works as you want it to, and then return it if you’re not satisfied. Bear in mind if you order a phone you must keep it in the box in order to return it smoothly.

The big risks are that you forget to cancel your contract or the network refuse to refund you for the phone if you’ve opened the box. But if you’re careful, it’s an easier way to test which network is right for you.

We’ll help you find cheap SIM only and phone deals using the links below.

Useful link: Compare SIM only deals | Compare phone contracts

You can predict indoor signal without using a SIM as well

Most people don’t want to go through that sort of effort just to know which provider is best for them. Luckily there are ways to get some pointers as to which will work best for you without needing a test SIM.

You can see each network’s predictions for their own indoor coverage on their network maps here. And the government regulator Ofcom have an indoor signal checking tool as well for each of them.

Which providers use each network...

There’s many providers on each network that give cheaper deals than their hosts while bringing the same signal to their customers. Here’s what providers use which network:

EE

BT Mobile, 1pMobile, Plusnet Mobile, (you may see masts marked as Orange and T-Mobile on mastdata.com).

O2

giffgaff, Tesco Mobile and Sky Mobile.

Three

iD Mobile, SMARTY, Superdrug Mobile (see may see masts labelled as Hutchinson on mastdata.com.

Vodafone

Virgin Mobile, Lebara, ASDA, Talkmobile and VOXI.

Ofcom’s tool is a bit limited in terms of data. We’d say it’s worth checking mastdata.com's database to find which masts are closest to you. It’s a paid service but you can get a free trial.

If you use a network’s coverage checker and order a free pay as you go SIM then you can get a comprehensive idea of which network will be the best for you.

Useful link: All about ordering test SIM cards

4G calling can help with indoor mobile coverage

As we’ve said with the causes of indoor blackspots, the lower the frequency the better it is for getting signal indoors.

There’s a tendency towards networks having less-optimal higher frequency 3G/2G signal while broadcasting 4G on lower ones. In the past, 4G was only used for data.

But now, most phones support 4G calling (VoLTE). So long as your phone is supported and your network offers it (not all do) you can make calls over 4G signal.

This means there are more frequencies for your phone to connect to calls with, and it’s good news for networks like EE and Three who broadcast their call signal on high frequencies, but their 4G on lower ones.

Networks with 4G calling: ASDA, BT Mobile, EE, iD Mobile, O2, Sky Mobile, SMARTY, Three, Virgin Mobile, Vodafone, VOXI

WiFi calling is also helpful if your network and phone support it

If you’re somewhere with poor indoor signal but a decent enough WiFi connection, you may also be able to connect to calls using WiFi in order to avoid indoor blackspots.

WiFi calling lets your phone connect to your network with WiFi connection instead of conventional phone signal. This lets you make and take calls as you usually would on your regular number.

While most providers support it now, not all of them do. Some that do support it only do so on a limited range of phones. We highly recommend checking if your model is supported by whatever network you’re thinking of joining.

WiFi calling can be even more useful than 4G calling in a way. You can make calls in places you couldn’t hope to get signal otherwise, such as on the Underground, as long as you have WiFi.

It’s certainly more convenient than buying an old-school mobile signal booster.

Networks with WiFi calling: ASDA, BT Mobile, EE, iD Mobile, O2, Sky Mobile, SMARTY, Three, Virgin, Vodafone, VOXI

Mobile signal repeaters have fallen out of favour recently

There was a time a few years ago when the only available cure for bad indoor mobile signal was to get a signal booster, a box that uses your home WiFi to create a mini 3G transmitter in your house.

It was only ever the main networks that offered these and you often had to complain or beg them to give you one. You might even have to pay for one (which we find a little unfair).

If your phone and network support WiFi calling, a signal booster is unnecessary. Vodafone used to sell their own signal booster called Sure Signal but that's now been withdrawn.

To be honest, we think it's only really worth going down the route of getting one if you've tried everything else on this page and still come up with no joy.

Three have extra technology to help with indoor calling

Three’s 3G signal is broadcast at a higher 2100 MHz frequency, making it the worst for getting through building materials among the major networks.

That doesn’t mean you’ll necessarily struggle with it. If you’re close to the mast you’ve got a good chance of getting good signal indoors. See Three's network map here and Ofcom/Mastdata to check their 3G in your area.

This weakness prompted them to lead the way for 4G Calling (VoLTE). Their 4G is broadcast on low frequencies, making it good at getting into buildings (read Three's instructions here to make sure your phone’s compatible).

Their WiFi calling scheme is one of the most flexible too, as you can make calls and send and receive texts over WiFi signal where some only let you call (read more here).

Three's current best SIM only deals

If you’re still not sure, you can always order a free SIM to test it out. If not, you’re still entitled to the same 14-day returns policy as you are with every other network, so you can test them that way too.

Useful link: Three's SIM only deals | Three's phone contracts

BT Mobile have 4G calling and WiFi calling to help indoors

BT use EE’s 2G/3G signal, which is broadcast on 1800 MHz and 2100 MHz. These are quite high frequencies, and not ideal for getting into buildings.

If your phone isn’t supported for WiFi and 4G calling (check the "Is my phone compatible?" FAQs here and here), you may get poor indoor call signal, though again this depends on distance to mast.

BT Mobile's current best SIM only deals

You can check your nearest masts and predicted coverage using Ofcom and mastdata.com methods above (and you can use BT's network map to check if you're near 2G and 3G masts).

They don’t let you order a free pay as you go SIM, but their pay monthly SIMs do let you try them for 30 days and then return them without charge if you’ve changed your mind.

Useful link: Our full BT Mobile review

EE offer extra network features to boost calls indoors

The best way to know for sure how good EE’s signal is to test a free pay as you go SIM. Put it in your phone and either call the top up line or get someone to call you on the new number (make sure 4G and WiFi calling are turned off).

As we saw with BT, EE broadcast on sub-optimal frequencies for getting indoors (1800 MHz and 2100 MHz). But now, 4G calling and WiFi calling are available on all of their plans.

What’s really good about this is that EE's 4G is broadcast on lower 800 MHz, meaning if you can get 4G you should be able to make high quality calls, even indoors. Check your phone is supported by EE here.

EE's current best SIM only deals

Do check on EE's network map and with Ofcom and mastdata.com to see what sort of signal coverage you can expect in your area.

Useful link: EE's 14-day returns policy

giffgaff use more optimal frequencies for getting indoors

Being on the O2 network means that giffgaff gets access to call signals mostly on a low 900 MHz frequency (though it can be 2100 MHz in some areas). This is better for getting into buildings.

As we’ve said, the lower the frequency the more likely you’ll get signal indoors. However, check your distance to their masts. If you’re very far away you might not get good signal indoors or outdoors.

giffgaff's current best SIM only deals

But, giffgaff still don’t have 4G or WiFi calling, so having a good call signal is a basic necessity for them. It’s odd they still don’t have it as most cheaper networks do these days (and O2 offer it themselves).

You can’t test giffgaff’s SIMs for free as you’ll need to top it up to activate it (you can get £5 free credit with a £10 top up here). For a truly free way to test see O2's PAYG SIMs.

Useful link: giffgaff SIM only plans | giffgaff phone contracts

iD Mobile's network features help with indoor call signal

iD Mobile use Three’s signal for calls, which are broadcast on frequencies that aren’t ideal for getting into buildings.

If you’re close to one of Three’s masts this shouldn’t be an issue (check via this page at idmobile.co.uk). But if you’re not, iD have features in place to help.

More and more budget networks are offering WiFi calling and 4G calling, but iD was among the first. What makes this even better is lots of their 4G is broadcast on the indoor blackspot busting 800 MHz frequency.

iD Mobile's current best SIM only deals

You’ll have to make sure that your phone is on iD’s supported models list. You can’t get free test SIMs from iD, but you can easily get one from Three if you want to see how their signal is in real life.

Useful link: iD Mobile's 14-day returns policy

O2 have better frequencies for calls inside with extra features

If you’re on a pay monthly deal with O2, you should never run into issues with indoor blackspots (unless you’re on the Underground without WiFi). Their pay as you go customers are much less well equipped, however.

O2 broadcast their signal on a low 900 MHz frequency, already making it one of the best for getting into buildings without extra technology.

But if you’re far away from the mast or have poor indoor signal for any reason, they have 4G and WiFi calling to help this out. Your best bet is to buy your phone with them as you’ll need special O2 firmware for it to work (see their guide here).

O2's current best SIM only deals

You also have to choice to order a free test pay as you go SIM card from O2. But you’ll only be able to test 3G/2G signal as 4G calling and WiFi calling aren’t offered on O2’s pay as you go.

Useful link: See what masts O2 have in your area

Plusnet Mobile offer nothing to help with indoor calling

When you’re out and about, being on EE’s excellent geographical coverage is definitely an upside for Plusnet. But they use high frequencies to carry phone calls which makes their indoor signal quite poor.

Unlike most networks and providers, Plusnet still don’t offer WiFi calling or 4G calling to help out with this. So you’re out of luck if you don’t live near a mast.

It’s still worth checking to see if you are near a mast or not. If you are then you shouldn’t run into too many problems (see Plusnet's network map).

Plusnet Mobile's current best SIM only deals

To be absolutely sure we recommend using a free SIM in real life to test. Plusnet don’t offer these, but you can order a free one from EE to test out the network and then jump on Plusnet if it works out.

Or you can use Plusnet's 14-day returns policy and cancel your SIM if you find that they’re just not the right network for you.

Useful link: Plusnet 12-month SIMs | Plusnet 30-day SIMs

Sky Mobile use good frequencies for getting indoors

Sky Mobile use O2’s signal to broadcast their 2G/3G phone signals, which are mostly on low 900 MHz frequencies which is pretty ideal for getting into buildings.

They also offer 4G calling, but make sure your phone is supported here. This means you’re much more likely to get good signal, even indoors.

For WiFi calling, Sky don’t support a massive number of phones and the only way to guarantee it will work is by buying your phone from them directly (see their help section here).

Sky Mobile's current best SIM only deals

You can see Sky’s prediction of indoor coverage on their network map here, which is always worth checking. Or you can check where O2’s masts are using Mastdata.

You can’t get a free SIM from Sky to test them out (you could get a free one from O2 directly and come back to Sky). But you could order a SIM and cancel it within 14 days if you find they’re not for you.

Useful link: Sky SIM only plans | Sky phone contracts

SMARTY's frequencies aren't optimal for signal in buildings

The higher 2100 MHz frequency that SMARTY uses via Three’s network should mean that indoor calling signal will be generally poorer on SMARTY than some other providers.

If you’re close enough to their masts, this shouldn’t actually be too much of an issue as you should still be able to get strong 3G. They’re another budget network that provide WiFi calling and 4G calling, so that helps a lot too.

SMARTY's current best SIM only deals

Use SMARTY's coverage checker here to see how their 3G signal is in your area. And use Ofcom and Mastdata's maps to see where the nearest mast is (you’re looking for Three/Hutchinson).

Even if you find that signal is poor in your area, we still recommend testing free pay as you go SIM card from Three (SMARTY only offer pay-up-front options) and turn off 4G and WiFi calling for your test.

Useful link: SMARTY's SIM only plans

Talkmobile: good frequencies but lacking extra technology

Talkmobile use the Vodafone network for phone signal, which gives them access to low 900 MHz frequencies, making them ideal for getting through building materials.

This is a necessity for Talkmobile as they don’t offer WiFi calling or 4G calling. This is one of the few differences between joining Talkmobile and Vodafone directly.

Talkmobile's best current SIM only deals

Talkmobile no longer offer pay as you go SIMs, meaning that option for testing their signal is out of the question. You can still order a pay monthly SIM and return it within the 14-day returns period.

Useful link: Talkmobile's SIM only plans

Tesco Mobile broadcast their 3G/2G on ideal frequencies for indoor use

Being on O2’s network means that Tesco Mobile can broadcast calls on their 900 MHz frequencies that do a decent job of getting into buildings.

That’s good news for them, because otherwise their lack of extra network features like 4G calling and WiFi calling would make getting signal indoors very difficult with them. If you’re far from a mast you may still struggle.

Tesco Mobile's current best SIM only deals

You can predict how indoor and outdoor signal coverage with their coverage checker here. It’s worth it to check Ofcom and Mastdata's maps as well for an overall more comprehensive view.

It’s nice that you can order a free pay as you go SIM card from Tesco to test with. Or you can cancel your contract within the first 14-days if you’re not happy (check full returns policy here first).

Useful link: Tesco SIM only contracts | Tesco phone contracts

Virgin Mobile offer extra features to help with indoor coverage

Virgin have recently moved over to using Vodafone’s network, meaning they get to broadcast their call signal on a low 900 MHz frequency, which should improve indoor signal coverage.

They also offer WiFi calling and 4G calling as provisions against poor indoor signal. This is great as it means almost all of their customers should be able to get good call signal, even those far away from their masts.

You can use their coverage checker to see what sort of signal strength you can get in your area (check for 4G and 3G/2G). You can use Ofcom/Mastdata too.

Virgin Mobile's current best SIM only deals

Unfortunately you can’t order a free test SIM as they no longer offer their pay as you go scheme. But you can still order a pay monthly SIM and return it if you’re unhappy.

Useful link: Virgin Mobile 14-day returns policy

Vodafone broadcast on ideal frequencies and offer extra features

Vodafone are one network that can offer most of their customers great indoor signal, as they have all the tools as their disposal for busting indoor blackspots.

Not only are their calls transmitted over nice-low 900 MHz on 2G and 3G, which is ideal for getting into buildings, but they also offer WiFi calling and 4G calling on all SIM only and phone deals (but not on pay as you go).

Vodafone's current best SIM only deals

All of that means you should always get access to decent signal indoors. Still check their signal strength in your area using their map here. You can always order a free test SIM too.

You can also benefit from the usual 14-day guarantee so you can send your pay monthly SIM back if you’re unsatisfied.

Useful link: Vodafone SIM only | Vodafone phone contracts

VOXI broadcast on naturally good frequencies for getting indoors

VOXI use Vodafone’s low 900 MHz frequencies for 2G/3G phone call carrying signal which is good for getting indoors. You’re getting the same advantages as on Vodafone too.

They also now offer WiFi calling and 4G calling, making up for their previous shortfalls. Now, if you find signal is poor indoors you can use these features to help with that.

VOXI's current best SIM only deals

There’s no way to order a VOXI SIM without paying first. But you can test how signal is in your area by ordering a Vodafone PAYG SIM (WiFi calling and 4G calling aren’t offered so don’t worry about that).

You can also use VOXI’s network map here to check call strength in your area. Or you can check where Vodafone’s masts are on Ofcom or Mastdata’s websites.

Useful link: Our full VOXI network review