Your Cell Phone’s SIM Card

Anyone who has a mobile phone has heard of a SIM card. These tiny memory chips hold the key to making your cell phone work. But have you ever wondered exactly what a SIM card does?

What Is a SIM Card?

The SIM card on your phone, more formally called a Subscriber Identity Module, is a small memory chip stores information about your phone account, such as your phone number, address book and text messages. Some SIM cards store more information that others. These cards are designed to help you easily switch phones when you get a new one. All you have to do is transfer the SIM card over, and you can activate your phone right away.

How Are SIM Cards Different?

There are many differences that you may encounter in SIM cards, including:

  • Size: All SIM cards are 0.76mm thick, but they can vary in length and height. Mini SIM cards are 25 mm by 15 mm. Micro SIM cards are 15 mm by 12 mm, and full-sized SIM cards are 85.6 mm by 53.98 mm. The full size models are usually too big to fit in phones.
  • Carrier: Most SIM cards are “locked” by a certain carrier, like Verizon or T-Mobile. You can buy unlocked SIM cards or unlock one yourself, but most people do not have that need.
  • Service: Some SIM cards are set up for international calling, prepaid plans or traditional wireless plans. You have to get the one that is appropriate for your account and usage.

Not every SIM card will work with every phone. Some can only be used in select devices. You should get a SIM card with your service plan, so all you will need to do is tell your carrier which phone you have. They will ensure that your new SIM card fits properly.

How to Replace SIM Cards

SIM cards can stop working over time, especially if they are removed a lot. The wearing will eventually make then illegible to your device. If you ever have to replace your SIM card, follow these steps:

  1. Contact your carrier and explain the problems you are having. They may be able to troubleshoot a different issue entirely.
  2. If the issue is with your SIM card, request a replacement card. Most carriers will provide this for free. If you are buying one from the store, simply find the one that works for your phone and make the purchase.
  3. Turn off your phone before replacing the SIM card.
  4. Find the SIM card slot in your phone. This will most likely be in the back, either around or behind the battery. Some phones have protective sleeves over their SIM cards that you will need to remove as well.
  5. Slide the old SIM card out, if there is one, and put the new one in.
  6. Activate your new card. You may be able to do this when your phone turns on, or you may have to call your phone service provider to do so. Make sure you hold onto the little plastic piece that the SIM card was attached to, as that may have information on it that your phone company will need.
  7. Test the phone and see if it works.

Note that these steps may vary from one phone to the next. Contact your service provider if you have any questions along the way.

Where to Get a New SIM Card

If you need a new SIM card, you should be able to get it from your current cell phone provider. If you do not have a store in your area, you can contact your provider’s main line to get in touch with someone who can help you. You can also buy SIM cards online from sites like Amazon and eBay, from retailers such as Best Buy, Walmart or Target, or a cell phone accessory supplier.

About John Oldshue

John Oldshue is the creator of He worked for over 15 years in television and won an Emmy award for his reporting. He covers long distance and cell phone topics for SaveOnPhone.