If you use a cellphone, you probably have heard of or installed a tiny card called a SIM card. You may have wondered, what does SIM stand for. SIM is an acronym for "subscriber identity module," and the SIM card is necessary to sign on to your carrier's cellphone network. If you switch carriers and keep the same phone, you usually have to get a new SIM card.
What Does SIM Stand For?
The subscriber identity module (SIM) is a computer memory card that you insert into your cellphone to allow the phone to sign in to your carrier's network or another network you might use while roaming.
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It contains information that identifies you, and it can also store information you save to it, such as your cellphone contacts. That feature can be useful if you use the same SIM card with multiple devices or if you want to move the SIM card and the associated data to a new phone.
SIM cards come in three sizes: standard SIM, micro SIM and nano SIM, in decreasing order of size. Different phones support different types of SIMs, with newer phones typically supporting the smaller cards. In some cases, you can use an adaptor to use a smaller SIM card with a phone that supports a larger card or pop a smaller SIM card out of a larger card to use in a phone that supports it. Check your phone's manual to see which SIM cards it accepts and how to insert and remove them.
Certain dual SIM phones support having two SIM cards in use at once. That can be useful if you want to connect to different carriers for different purposes. For example, it may be cheaper to call phones on one carrier's network from that carrier, or one carrier may offer a better voice plan while a competitor offers a better data plan.
If you travel internationally, you can often save money by buying a local SIM from a carrier in the country you're visiting. Just make sure that your phone is not SIM-locked to your home carrier, meaning it won't accept SIM cards from other carriers. Some carriers provide you with assistance in unlocking your phone or give you a code you can enter to unlock it.
The Rise of eSIM
Some of the newer phones, including the iPhone X, support a new standard called eSIM, or electronic SIM. This enables you to store SIM data on a chip within the phone rather than on a separate card.
The standard is still evolving, so you may not be able to use eSIM with your carrier even if your phone supports it. Check with your carrier to see if it's available.