ITS SIMCARD

A subscriber identity module or subscriber identification module (SIM) is an intergrated circuit chip that is intended to securely store the international mobile subcriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices (such as mobile phone and computers). It is also possible to store contacts on many SIM cards. SIM cards are always used on GSM phones; for CDMA phones, they are only needed for newer LTE-capable handsets. SIM cards can also be used in satellites phone .
The SIM circuit is part of the function of a universal integrated circuit card  (UICC) physical smart card, which is usually made of PVC with embedded contacts and semiconductors. "SIM cards" are designed to be transferable between different mobile devices. The first UICC smart cards were the size of credit and bank cards; the development of physically smaller mobile devices has prompted the development of smaller SIM cards, where the size of the plastic carrier is reduced while keeping electrical contacts the same.
A SIM card contains its unique serial number (ICCID), international mobile subscriber identity (IMSI) number, security authentication and ciphering information, temporary information related to the local network, a list of the services the user has access to, and two passwords: a personal identification card  (PIN) for ordinary use, and a personal unblocking code (PUK) for PIN unlocking.

SIM cards store network-specific information used to authenticate and identify subscribers on the network. The most important of these are the ICCID, IMSI, Authentication Key (Ki), Local Area Identity (LAI) and Operator-Specific Emergency Number. The SIM also stores other carrier-specific data such as the SMSC (Short Message Service Center) number, Service Provider Name (SPN), Service Dialing Numbers (SDN), Advice-Of-Charge parameters and Value Added Service (VAS) applications. (Refer to GSM 11.11)
SIM cards can come in various data capacities, from 8 KB to at least 256 KB. All allow a maximum of 250 contacts to be stored on the SIM, but while the 32 KB has room for 33 mobile network code (MNCs) or "network identifiers", the 64 KB version has room for 80 MNCs. This is used by network operators to store information on preferred networks, mostly used when the SIM is not in its home network but is roaming. The network operator that issued the SIM card can use this to have a phone connect to a preferred network, in order to make use of the best commercial agreement for the original network company instead of having to pay the network operator that the phone 'saw' first. This does not mean that a phone containing this SIM card can connect to a maximum of only 33 or 80 networks, but it means that the SIM card issuer can specify only up to that number of preferred networks; if a SIM is outside these preferred networks it will use the first or best available network.