A SIM card, or Subscriber Identity Module card, is a small plastic card with an embedded circuit chip that stores identifying information on a mobile device. The SIM card is a type of smart card that securely stores an international mobile subscriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices. SIM cards store network-specific information used to authenticate and identify subscribers on the network, including the ICCID, IMSI, authentication key (Ki), local area identity (LAI), and operator-specific emergency number. Other carrier-specific data such as the SMSC (Short Message service center) number, service provider name (SPN), and service dialing numbers (SDN) are also stored on the SIM card.
SIM cards are essential to the working of most cell phones, as they link a physical device with the owners account, making it possible to route calls for individuals to the right device and allowing phone companies to accurately measure utilization and charge subscribers for their service. Without a SIM card, some phones would not be able to make calls, connect to internet services such as 4G LTE and 5G, or send SMS messages.
SIM cards are removable and have anywhere from 32 KB to 128 KB of storage. An individuals SIM card can be a target for hackers because the SIM card has indirect access to a persons email, banking information, or social media accounts. However, some phones are now equipped with eSIMs, which are digital SIM cards that dont require any physical chip to be embedded in the device.
