Discovery Hub

What is a SIM Card and How Does it Work? 


January 31, 2019

If you’re looking to take your phone to a new cellular provider, you’ll need to know about SIM (or, “Subscriber Identity Module”) cards. These are tiny, removable “chips” that are essentially the “brains” of your device. Just like your brain, the SIM holds everything unique to you on the phone. In the case of a phone that includes your number, your contacts, and some of your text messages. A SIM has a small amount of memory that stores these details, so if you switch your SIM into a new phone, all that information typically comes with it. This also makes it super easy to switch carriers.

Learn more about the benefits of switching to Xfinity Mobile.

Here’s everything you need to know about SIM cards.

Does every phone have a SIM?

All new smartphones have SIM cards (some older phones may not), but you can’t always switch SIMs between phones because there are two types of cellphone technology: GSM (Global System for Mobiles) and CDMA (Code Division Multiple Access). GSM phones use SIMs exclusively, whereas CDMA phones also rely on internal hardware to identify your phone.

Which phones allow you to switch SIMs?

All GSM phones allow you to switch SIM cards between them, but a GSM SIM will not work in a CDMA phone. Today, Verizon, Sprint, and US Cellular use CDMA, while AT&T and T-Mobile use GSM. You may have noticed that your Verizon or Sprint phone has a SIM in it, but that is to allow access to the LTE network. These SIMs don’t contain the same information as GSM phone SIMs, which is why you can’t swap them out.

However, you can still transfer your old CDMA phone to a new network even without a SIM. As long as the phone is paid for, just contact the company and have it “unlocked.” Soon this may not be an issue, as Verizon is planning to discontinue CDMA by the end of 2019, making it easier for AT&T, T-Mobile, and Verizon devices to be compatible.

What about iPhones?

Apple made all iPhones, no matter which wireless carrier they use, compatible with both GSM and CDMA. This means you can switch SIMs easily between any iPhone newer than an iPhone 4s. However, an iPhone can still be “locked” to a carrier, so you may need to get it unlocked when transferring to a new carrier.

Can I use my phone without a SIM?

If your phone doesn’t have a SIM card in it, it most likely will not work as a phone, but you could still connect to WiFi and use all the apps and Internet functionality it contains. Think of it like an iPod Touch, or an iPad.

Can I put my SIM into another phone?

Yes, if your phone is a CSM phone or an iPhone you can pop out your SIM card, put it in a different phone, and make a call using your contacts and your phone number. This can be very useful in an emergency, or when you lose or break your phone.

Are all SIMs the same size?

Most smartphones use Nano Sims, but you still may come across Mini SIMs and Micro SIMs. It’s possible to cut a larger SIM to fit into a smaller SIM slot, and there are special tools available to do this. Just make sure it's only the plastic that’s cut, not the metal chip part. iPhone models 5 or newer use Nano SIMs, Samsung Galaxy S4 and S5 phones use Micro SIMs, and Galaxy S6 models or newer use Nano SIMs.

Can I have more than one SIM?

Yes, if you travel between countries frequently, you might want to have a SIM card from each country, which you can switch out upon arriving there. This will allow you to have a local phone number and your local contacts on your phone immediately, so you can make calls right away.

Keep in mind that SIMs don’t store iMessages, photos or any of the data in your apps or other Internet-related parts of your smartphone. If you want to back up that data to transfer it to a new phone, you’ll need to use a cloud backup service, or back it up to a computer. Learn more about the devices and storage options available with Xfinity Mobile.

iPhone 14 Pro

iPhone 14 Pro

Experience iPhone in a whole new way with Dynamic Island and Always-On display.

Shop iPhone