SIM (Subscriber Identity Module) is a small card that contains mobile network subscriber's account information. This allows the phone using the card to attach to a mobile network. The SIM card is most commonly associated with GSM and UMTS mobile networks. Moving a SIM card from one phone to another allows a subscriber to switch mobile phones without having to contact their mobile network carrier. SIM cards can also be used by a phone to store limited amounts of data, such as phone numbers and text messages.