Mobile SIM companies

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.

A SIM card contains its unique serial number, international unique number of the mobile user (IMSI), security authentication and ciphering information, temporary information related to the local network (also temporary local id that has been issued to the user), a list of the services the user has access to and two passwords (PIN for usual use and PUK for unlocking).
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 length of 15 mm, and a thickness of 0.76 mm. However, most SIM cards are still 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).

http://www.telestial.com/
http://www.wisegeek.com/what-is-a-sim-card.htm
http://intouchsmartcards.com/

http://www.telestial.com/sim_cards.php