What exactly is a SIM card? You know, the little piece of plastic found in the metal pullout on the right side of your iPhone or cellular iPad? The short answer: it's a Subscriber Identity Module, a small circuit board in most modern phones to communicate with your carrier. Practically speaking, it is a middleman between two pieces of hardware: the phone's baseband chip, and your carrier's cell towers, allowing the two to communicate.
SIM cards allow you to travel between phones with your phone number and data plans; when you upgrade to a new iPhone, the SIM card travels (as long as it is the same size). But while SIM cards themselves may be portable, they only work in certain phones. We'll explain why in a bit.
A SIM card is internationally identified by its Integrated circuit card identifier (ICC-ID), which is engraved on the body of the card. It is also identified by the carrier with its International mobile subscriber identity (IMSI). Essentially, these two numbers tell the carrier that your phone is allowed to operate on its network and, once connected, should be billed for certain features. Beyond identification, SIM cards have several other functions.
What is a SIM card?
SIM cards have evolved a lot over the years. While they have maintained a relative thickness of just under 1mm, their surface area has steadily decreased, from the credit card-sized plates used in the earliest cellphones to the nanoSIMs of today's devices.
The nanoSIM is the SIM card's fourth size standard since its inception. Designated as 4FF, or fourth form factor, it measures 12.3mm x 8.8mm x 0.67mm, a reduction of over 42 times from the SIM's inception. Most people, however, are more familiar with the 2FF SIM card, known more readily as the Regular SIM card. At 25mm x 15mm x 0.76, it is 3.4 times larger than today's current SIM cards, which offer the same features in a much more compact package. Some manufacturers still choose to use the microSIM format which, at 15mm x 12mm, is only slightly larger than the nanoSIM.
Many carriers offer branded SIM cards with cutouts for all three modern sizes, so users can choose which version they want to insert depending on their device. There are also adapters so nanoSIM cards can fit into slots meant for microSIM or Regular SIM cards.
How does a SIM card work?
Often, a SIM card is provided with the purchase of a phone by your carrier and it is used to store data about your account. Because it has a small amount of memory and a very low-powered processor, the SIM card not only enables communication between the phone and its carrier, but stores information such as phone numbers, security data and more. In recent years, carriers have begun using specialized SIM cards with so-called Secure Elements to store credit card credentials in order to facilitate mobile payments.
Bonus Tip: SIM cards are transferrable. If your iPhone runs out of power and you desperately need to make a call or connect to the internet, you can just swap the SIM into another iPhone and use it with your minutes and data bucket.
What are the benefits of a SIM card?

SIM cards are the unseen magicians of today's smartphones. They make connecting to networks and switching phones as easy as removing a small metal tray.
If you buy a new phone, you can simply insert your existing SIM and keep on using your existing service as long as the new phone isn't locked to a different carrier. Likewise if you travel internationally you can just buy a SIM card from a local carrier — as long as your phone isn't locked to a carrier.
What are the drawbacks of a SIM card?
SIM cards are fairly simple. They have practically no function without an accompanying smartphone and are increasingly reliant on third-party apps to address functionality, such as connecting to multiple networks or using more than one phone number.
And because they are simple pieces of plastic, their benefit is largely subject to the will of the carrier. If you buy a phone that is locked to one carrier, putting a different carrier's SIM requires an unlock code, which is often expensive to obtain and confusing to enter.
Typically the SIM lock is in exchange for a subsidized phone (so you can't buy a cheap phone on one carrier then switch over an use another before you've paid back the subsidy over the life of your contract).
These days, Apple sells the iPhone SIM-unlocked in most countries, including the U.S., Canada and the U.K. While customers have to pay more upfront for an unlocked version of the iPhone, they can use the phone with any compatible SIM card, as long as the phone works with that carrier's network. The latest iPhone, the 6s, supports 23 LTE bands, which comprises most of the world's carriers.
What is Apple SIM?
In 2014, alongside the iPad Air 2 and iPad mini 4, Apple unveiled Apple SIM, a way for the company to assert some control over its devices' connection to various carriers.
By working with select providers in the U.S. and U.K., Apple SIM allows iPad owners to connect to whichever network they want — provided that telco has worked with Apple. In the U.S., those carriers include Sprint and T-Mobile; in the U.K., EE. The beauty of Apple SIM is its ability to interface with multiple networks; when visiting foreign countries, partners GigSky, AlwaysOnline Wireless and au work with local providers to offer per-day or per-megabyte bundles.
Most recently, with the 9.7-inch iPad Pro, Apple began embedding its Apple SIM in its tablet line, allowing the use of both Apple's partner networks as well as local carriers, such as AT&T and Verizon in the U.S.
Reader comments
What is a SIM card and what does it do?
Speaking of iPhone unlocks, is this real?
http://www.gsmliberty.net/shop/apple-iphone-unlock-p-1477.html
The service says that it'll make it be unlocked even through firmware upgrade. Looks almost like they'll make it be factory unlocked.
I've used them to unlock other phones before, so I trust them, but I didn't know this was possible. Is it?
I have an iPhone 4 for which my employer pays - My wife has an iPhone 3g for which we pay - both are with AT&T. If I'm reading your article correctly, I could conceivably exchange the SIMs so that I could use the 3G and she could use the 4, assuming that I had backed up each phone and done a full reset. Is that correct?
Yep! Thats exactly correcy sir :D
You'd need a SIM jacket to use the same SIM in iPhone 3GS and iPhone 4 since iPhone 3GS uses the larger mini-SIM.
I like how everything is base on the iPhone. Like the first couple of words "New to IPhone" just to let you know, not only the iPhone has a sims cards other phone has it too...
In case you didn't notice, this is a website devoted to the iPhone, iPad, etc. They don't mention any other phones that use sims? Go figure!
I found it especially funny because a large number of the people that are probably "new to the iPhone" at this time probably don't have SIM cards in their iPhones...
The**
Don't worry, a quick trip back to kindergarten can help ya with that.
i could picture Arnold Schwarzenegger saying the title of this article.
:lol:
Yes! To funny Dominick.
Just my thought as well. "TiPb Answers: Who is your daddy and what does he do?"
I lift dings op and poot dem down.
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Beware, GSMLiberty is a SCAM (search the web for it).
Let me tell you how legal unlocking works. (I live in France)
After the 6 months engagement period (or paying the unlocking fee) you call the operator and give them your IMEI.
The operator transmits the IMEI to Apple for inclusion in their database for unlocked iPhones.
Then the operator contacts you when it receives the clearance from Apple.
And finally you'll have to restore your iPhone and iTunes will say " Your iPhone is now unlocked ". (Some say, you simply have to put the SIM of another operator and sync)
And it is not permanent, each time you restore the iPhone, iTunes will redo the unlocking.
Great and informative article, Georgia! I've known about SIM cards for years, but never actually had a phone that employs one until I got my iPhone; your article helped me to better understand the benefits of the SIM card :)
Also, reading your article brings a thought to mind: if signing up for a 2-year contract with a carrier to subsidize the price of a phone, why doesn't the carrier lower the plan price after two years when the phone has already been paid for? Aside from the obvious answer that the carrier wants to continue taking my $$$, what other reason/explanation is there?
Interesting.... I read somewhere that one of the carriers is trying to reduce the buying cycle....
The best way to do that is to lower the price at least a little when its paid off. I'll stick with my phone longer.
But also note. Some carriers require a 2 year contract even if you don't buy a phone. Or you pay pre-paid prices. (which for the carrier is higher) or you could use the carriers spin off prepaid companies i.e. virgin and boost are both actually owned directly by the big guys. But they offer prices that are lower kinda like 3rd party companies that buy minutes in bulk and offer prepaid options. Some are decent. But in general service, and features lack.
So, I just bought a 32g 1st generation Ipad. It doesn't come with a SIM card. Is it supposed to? Will I need to purchase one in order to use the Ipad?
Not that I know of,no were to plug in
Here's my question and I know it's basic. I want to order the iPhone5, but currently have a 3G. The SIM card in not compatible so it's not just a matter of switching it out....so when I get the iPhone 5, how do I actually activate the cellphone-phone/phone with my old phone number ? Do I need to take it to an AT&T store? I know no old SIM cards are compatible with the new iPhone so there must be a way to set it up without storming carriers' by the millions.
I'd like to sell my unlocked iPhone 4S on eBay. It was locked to AT&T, but I requested an unlock from them. Should I include the AT&T SIM card in the phone? Or is it better not to?
Sorry. I registered to try and find out more about smart phones and stuff. I've got an ex-contract iphone 4 and after a lot of unlock faffing am about to give up.
I was not able to understand the drawbacks of sim card properly.
I would be obliged if anyone helps me.
I have an iPhone 6S Plus that replaced a iPhone 5S (which I still have). I want to use it like you would an iPod Touch with AT&T's NumberSync. With my 6S+ as the primary, it works well with my MacBook Pro. When I attempt to hook the 5S up to WiFi calling, it messes things up after which I have to reactivate it on my MacBook. The old AT&T SIM is in the 5S -- could it be something about the SIM that's interfering with connecting to to NumberSync (WiFi calling)..?
Yes, remove the sim from the 5s. It's not needed for wifi only modes.
Thanks -- I've tried that more than once. Different than an iPod Touch, without a SIM I can't access the Settings/Phone screen to turn on WiFi Calling -- get an error that there 'is no SIM inserted'. I can log into FaceTime and iMessage, but not NumberSync WiFi Calling...
I'll be in Canada for two weeks starting Sunday. What do I need to do to avoid extensive charges for when I want to send photos to friends and family?