A SIM card is a microchip around the size of a postage stamp holding all of a mobile phone’s configuration information, for example the mobile phone number and call plan. Without a SIM card, a mobile phone will not work. SIM stands for Subscriber Identity Module. Once a SIM card has been activated it is permanently locked to the particular network, though not locked to a particular phone. That means you can easily change mobile phone handsets but keep your phone number, your friends’ phone numbers and any saved messages or photos.