A SIM card, also called a subscriber identity module or subscriber identification module, is a small memory card that contains unique information that identifies it to a specific mobile network. With this card, subscribers can use their mobile devices to receive calls, send text messages or connect to mobile internet services.
How do SIM cards work?
Some phones require a SIM card to identify the owner and communicate with a cellular network. For example, an iPhone on the Verizon network requires a SIM card so that Verizon knows who the phone belongs to and that they are paying for the plan, and also so that certain features work.
This is important in resale situations, where a used smartphone is missing a SIM card. You can still use the device's camera or Wi-Fi functions, but you won't be able to send text messages, make calls, or connect to a carrier's mobile internet network.
Some SIM cards are cellular, meaning that when they are transferred to a new or upgraded phone, the phone number and carrier plan details are transferred as well. If your phone runs out of battery and you need to make a call and you have a spare lying around, you can pop the SIM card into the other phone and use it straight away.
