SIM card description & definition

The SIM - Subscriber Identity Module - is a smart chip card, with the size of a postage stamp. The SIM card contains a microprocessor chip, which stores unique information about your account, including your phone number and identifies you to the network. So it is NOT your cell phone which determines your telephone number, but it is your SIM card. Instead, subscribers activate their phones by inserting SIM cards. Therefore, every time you change your SIM card, you are getting a new telephone number.
A SIM card is actually a tiny computer in your phone. Current SIMs typically have 16 to 64 kb of memory, which provides plenty of room for storing hundreds of personal phone numbers, text messages, value-added services and important for us: position data (coordinates) of tracked animals.

The embedded circuitry located on the SIM card stores information about the services that are available to you as a local subscriber. The SIM card identifies you to the network and contains a microprocessor chip, which stores unique information about your account, including your phone number. The SIM card, provided by a local GSM cell phone operator, snaps into the back of a GSM cell phone to make you a local subscriber.

SIM cards are available on a subscription basis, you sign a contract of a provider and get a bill every month or they are available on a prepaid basis, in which case you buy airtime, as you need it. In other words, you can borrow almost any other GSM phone and insert your own SIM card and make calls as usual. These calls will then appear on your own phone bill, as if made with your own phone. In the world of SIM cards you are not phone dependent, all you need is your own SIM card and almost any GSM phone.

Since - in most cases - the SIM card is removable by the user, it makes it possible to carry your mobile subscription and data through different types and generations of GSM phone. The interfaces between the mobile handset and the SIM card are fully standardized and there are already specifications in place for 3rd generation handsets and SIMs.

Most GSM mobile/cell phones are sim unlocked and will work with any sim card from any carrier in the world. Sim cards may be purchased online for a growing number of select international destinations.
Most GSM phones sold in the U.S. are locked to a single SIM card. Stick another SIM card into a locked phone and it won't work.
Where is the cell phone number of my SIM card?The phone number of your SIM card is usually located somewhere on the packaging. The number printed directly on the SIM card is a SIM card number and is not the phone number.