A SIM card lives inside your cell phone and is essentially your phone’s ID. It stores contacts and other data, and your phone won’t work without it. A SIM card works by connecting with your mobile phone and your carrier’s cell towers, so the two can communicate. When you upgrade your phone, you bring your SIM card with you to keep your phone number, data plans and more; but since most SIM cards are designed to work only with certain brands and carriers, they won’t fit just any phone. Whether you’re shopping for a new smartphone or refurbished cell phones, check out Best Buy for great deals every day.

If you love to travel and want an international SIM card, or you simply don’t want to commit to a carrier, an unlocked phone might be right for you. Unlocked phones are packed with all the features you want, plus you can move from carrier to carrier whenever you like, making it easier to transition internationally. Unlocked phones use an interchangeable SIM card that can connect to multiple Global System for Mobile (GSM) networks, like Sprint and Verizon in the United States. Prepaid phones work in a similar way, in that you can avoid an expensive wireless plan by paying month-to-month, but they are compatible with only one specific carrier.

From an AT&T SIM card, T-Mobile SIM card, or Verizon SIM card to other prepaid SIM cards, they usually come in one of three popular sizes: Regular SIM cards, micro SIM cards and nano SIM cards. A nano SIM card offers the same features as larger cards do, but in a more compact package. Many phones are equipped with cutouts for all three sizes, so you can decide which version you want. If your phone fits only one type of SIM card, you can try using an adapter, which allows nano SIM cards to fit into micro SIM or regular SIM slots.