What is SIM ?

And SIM card, we’ve got a newfound respect for you sia!

It’s how you chat to your friends, it’s the reason you remember every single person’s birthday. It’s your portable camera. These days it can even be a handy replacement for a printed-out ticket or debit card.

But, have you ever stopped to think that it’s actually the SIM card hiding inside that makes all the good stuff possible?

That tiny chip is a total game-changer, and knowing all the tricks of the trade can help you get the most out of your phone. Here are the 10 most frequently asked SIM card related questions, and their answers.

Your SIM card is actually a bit like the brain of your mobile phone so… no. If you had a serious accident with your phone, killed the screen perhaps, or bent the case so impressively that it was beyond repair, you could transplant the SIM into a similar handset and within seconds have the same phone number, and access to all your contacts – and depending on the precise model of handset, quite a few other handy bits of data too.

A SIM card is one great example of smaller being better. As technology has advanced, SIM cards have got smaller without being any less powerful. There are three main sizes in circulation at the moment – standard, micro and nano SIMs.

Standard SIMs tend to only be used in older devices, such as iPhones up to the 3GS or that weird 90s-looking brick thing you saw in your Grandma’s handbag.

Micro SIMs are still fairly common: acting as the brains of iPhone 4s and older iPads, for example. Most micros are now being replaced by the nano SIM, though. If you’ve got an iPhone 8 or newer, or even one of the newer HTC Phones, this is what you’ll need.

There are adaptors to fit the smaller, newer, SIM cards into the older, bigger phone slots – and some fearless people will even trim down the bigger SIMs with nail scissors to make them fit newer phones – but migrating your number and data is such a painless process you’re probably better off doing it by the book

Ever wondered what SIM stood for? No, nor us honestly. But knowledge is power, and you never know when these things will come up in a pub quiz. So, SIM stands for Subscriber Identity Module. Amazing, eh?

It’s not something that you’ll do every day, but if your phone’s battery was completely flat and you desperately wanted to call someone to let them know where you were, you could borrow a phone from someone, pop in your SIM card (as long as they have the same size SIM), and call one of your own contacts.

Because who on earth has memorised phone numbers off by heart? Better yet, as far as the person you’re calling is concerned, they’ll think the call’s coming from your phone. So you need never suffer the embarrassment of admitting that you forgot to charge it before setting off on that long trip.

This can also be super helpful if you have one of our SIM only deals, as it means you can simply transfer your SIM between handsets with exactly the same contract, and no hassle.

If you travel internationally a lot, maybe for business or because you’re insanely rich, it’s worth considering a second phone with a local tariff. Except if the countries you’re visiting are part of Three’s Go Roam network, of course

Unless your two handsets use compatible SIMs, you might need the contact info or text messages from one phone moved to the other. That’s where a SIM reader can come in handy. It’ll back up the data on your SIM card via USB and let you copy it freely.

Some more cautious people might like having a periodic backup of what’s on their SIM anyway. It’s just comforting to know that all the info is there.

Can the police or security services track you through your SIM card, like they do in the films? Well, boringly or mercifully, depending on your outlook, it’s a lot more complicated than that. All that can be absolutely determined is which mobile phone mast you’re connected to.

Normally, that’ll be the nearest one, but if you’re in an environment where the local mast is very busy – say at a football match or in a crowded train station – your call can be “handed off” to the second-nearest mast, or even the third.

It’s all very techy, but while tools such as Apple’s “Find My iPhone” and Android’s “Find My Device” can narrow locations down quite impressively using a mixture of mobile signal and GPS, your exact location will be a private matter for a good few years yet.

Your SIM card packs all your contacts, text messages and subscriber information into 64 kilobytes of memory. That’s a laughably tiny amount of memory, but it does the trick. For reference, the Apollo Guidance Computer used for the first Moon landings also had 64KB of memory. So, you’re in pretty good company.

Most smartphones will offer 64GB of memory for all the extra stuff like the apps, and your photos and such. That’s a million times as much, even more reason to look at our SIM only plans. If you’re confused about your GBs and KBs, check out our smartphone storage guide.

When you’re travelling, your SIM is the only thing that negotiates with overseas mobile service providers and gets your phone working in the time it takes for you to get off the plane and collect your bags from the airport carousel.

At Three, we hate the fact that people feel too scared to use their data services abroad. A study we conducted found that 20% of holidaymakers don’t like being without their phone on holiday, yet 1 in 5 switched data roaming off and 10% admitted to being laden with charges.

Now we’ve established that your SIM card is a pretty important little fellow, keeping him secure suddenly feels a lot more important. Your SIM has a separate security code to prevent it being used in another device, but the default ones are, to say the least, basic.

Give your SIM a more guess-proof code. On an iPhone it’s under Settings > Cellular > SIM PIN, but most handsets will let you set a unique SIM PIN.