A Subscriber Identity Module (SIM) on a removable SIM Card securely stores the service-subscriber key (IMSI) used to identify a subscriber on mobile telephony devices (such as computers) and mobile phones. The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone or broadband telephony device.

SIM cards are available in two standard sizes. The first is the size of a credit card (85.60 mm × 53.98 mm x 0.76 mm). The newer, more popular miniature-version has a width of 25 mm, a height of 15 mm, and a thickness of 0.76 mm. However most SIM cards are supplied as a full-sized card with the smaller card held in place by a few plastic links and can be easily broken off to be used in a phone that uses the smaller SIM.

The first SIM Card was made in 1991, with Munich smart card maker Giesecke & Devrient selling the first 300 SIM cards to Finnish wireless network operator Elisa Oyj (formerly Radiolinja).

Each SIM Card has a unique SIM Serial Number (SSN, also known as ICCID). The exact format of the SSN differs according to the mobile operator, however the following is constant:

• The first 3 digits represents the Mobile Country Code (MCC).
• The next 2 digits represent the Mobile Network Code (MNC, i.e. the mobile operator).
• The next 12 digits is the number, from which some operators use the two left most to indicate the Home Location Register.
• The 20th digit then is a checksum.

The numbering of the ICCID is based on International Standard ISO/IEC 7812. The maximum length of the visible card number is 20 characters; 19 digits are preferred, but telecommunication network operators who are already issuing Phase 1 SIM cards with an identification number length of 20 digits may retain this length. The number is composed of the following subparts:

Issuer Identification number (max. 7 digits)

• Major Industry Identifier (MII), 2 digits, 89 for telecommunication purposes.
• country code, 1-3 digits, as defined by ITU-T recommendation E.164.
• issuer identifier, variable.

Individual account identification

• individual account identification number.
• parity check digit.

W-SIM is a SIM card which also integrates core cellular technology into the card itself.

A Virtual SIM is a mobile phone number provided by a wireless carrier which does not require a SIM Card to terminate phone calls on a user's mobile phone.

Usage In Mobile Phone Standards

The use of SIM cards is mandatory in GSM devices. The equivalent of a SIM in UMTS is called the Universal Integrated Circuit Card (UICC), which runs a USIM application, whereas the Removable User Identity Module (RUIM) is more popular in CDMA-based devices. Many CDMA-based standards do not include any such card, and the service is bound to a unique identifier contained in the handset itself.

The Satellite phone networks Iridium, Thuraya and Inmarsat's BGAN also use SIM cards. Sometimes these SIM cards work in regular GSM phones and also allow GSM customers to roam in satellite networks by using their own SIM card in a satellite phone.

The SIM card introduced a new and significant business opportunity of mobile telecoms operator/carrier business of the MVNO (Mobile Virtual Network Operator) which does not own or operate a cellular telecoms network, but which leases capacity from one of the network operators, and only provides a SIM card to its customers. MVNOs first appeared in Denmark, Hong Kong, Finland and the UK and today exist in over 50 countries including most of Europe, USA and Canada, and Australia and parts of Asia and account for approximately 10% of all mobile phone subscribers around the world.

On some networks the mobile phone is locked to its SIM card such as on the GSM networks in the USA. This tends to happen only in countries where mobile phones are heavily subsidised, but even then not all countries and not all operators; such as in the UK, typically, most phones with subsidies are not SIM-locked. In countries where the phones are not subsidised, such as Italy and Belgium, all phones are unlocked. Where the phone is not locked to its SIM card, the users can easily switch networks by simply replacing the SIM card of one network with that of another while using only one phone. This is typical for example among young users who may want to optimise their telecoms traffic by different tariffs to different friends on different networks. It is called the "SIM card switch".