There's a very good chance your phone has a SIM card right now, especially since LTE networks have almost completely supplanted CDMA networks. Those cards allow your phone to connect to your cellular network for calls, texts, and data access, but how exactly do SIM cards work? What's so special about those little SIM cards that enable your phone to access a wireless network? We'll explain.
What is a SIM card?
A SIM card is essentially an authorization or registration “key” for a phone to talk to a wireless network. That SIM card and its unique serial number (the ICCID, or integrated circuit card ID) is registered with a cellular network and then tied to a phone number. So, the SIM card in your phone has a unique ID number, and your carrier has that ID registered with your personal phone number. Any phone that your SIM card is placed into can access that network based on your current plan and available features.
This also makes it very easy to change phones. When you get a new phone, you don't necessarily need to contact your carrier or any other manufacturer; simply pop your SIM card into the new device, and it will be registered on the network so you can use it. There are still some catches about whether that new phone supports all the bands your carrier uses, but the SIM card won't be the limiting factor there.
Why do you need a SIM card?
As we mentioned previously, your SIM card is the key to unlocking access to your carrier's network. Without authentication, you cannot access wireless services.
You might remember back in the day (especially in the United States) when CDMA was more prevalent, and Verizon and Sprint phones didn't use SIM cards and typically couldn't accept them at all. Those CDMA-compatible phones were explicitly built to use those carriers' networks, and the authentication key was built into the phone without needing a SIM card. This had some advantages and disadvantages, but it did tend to lock devices to specific networks and required you to call up your carrier to change phones when you got a replacement.
LTE networks are built using GSM and HSPA standards rather than CDMA standards. Therefore, they require SIM cards. Since most major carriers and countries around the world have adopted LTE over rival technologies like WiMAX, SIM cards are pretty ubiquitous these days.
But what about eSIM?
You might have a phone that supports eSIM, especially if you've purchased it in the last few years. This takes the physical SIM card that you're used to and builds it directly into the phone, complete with its unique ID number. However, this kind of SIM card is reprogrammable so that you can swap it around from network to network, and it can't be physically damaged since there's no plastic card or moving piece to break.
When you want to switch phones, you simply update the SIM number with your carrier to your phone's eSIM number along with the new device's IMEI. This adds an extra hurdle compared to just moving SIM cards from one phone to another, but it does prevent theft and normal wear and tear that you'd get from an old fashioned SIM card. Additionally, this makes it easier to have two phone numbers on one phone since the eSIM will be assigned its own number, and the physical SIM can have a second line.
Do note that this isn't quite like CDMA, where the phones had to be built to spec for certain networks.
Can I use my SIM card for storage?
SIM cards technically have some storage space. They're made to hold specific information to help your phone connect to a carrier, after all. Unlike microSD cards, though, they don't hold much.
Your typical SIM card isn't likely to have over 1MB of storage, which means you'd be lucky to fit anything besides contacts and text messages on it. Some phones do still support storing your contacts on a SIM card, but since SIM cards can and do go bad (sometimes without warning!), we'd fully recommend using cloud backups or an actual storage device.
Your SIM card should only be a concern when you're changing phones or having connection problems with your carrier. You do technically use it every day, but generally, they're made to set and forget until you're ready for a new phone.
