SIM is abbreviated as a subscriber identity module or subscriber identification module. It is an integrated circuit. It is a key and used to find the subscribers on mobile telephone devices. The main work of SIM card is transferred between various mobile devices. SIM card has unique serial number. It is used for security authentication, temporary information, ciphering information and related to the local network. SIM number is unique. You can give two passwords namely a personal identification number is used for ordinary use and a personal unblocking code is used for PIN unlocking. SIM card was made in 1991. There are three operating voltages SIM cards available such as 5 V, 3 V and 1.8 V. The operating voltage of the first SIM cards was established in 1998. SIM card is available in two types such as GSM and CDMA. SIM cards are used for making phone calls and sending messages.