Mobile phones have today become a necessity of modern day living. It is the most popular and easy way of remaining connected all the time with your family, relatives, friends, business associates, clients and customers, irrespective of distance. Whether it is in the same city or on foreign shores, the mobile phone, being portable, can be carried anywhere and everywhere. But, without the SIM card, this instrument is absolutely useless.

The SIM (Subscriber Identity Module) card is very small, even smaller than a thumb impression, but without it your mobile phone is a worthless and useless piece of plastic. The card is a data bank for the mobile phone and without it the user cannot make or receive calls, send or receive messages listen to music and click photos. It would not be incorrect if we compare the SIM of a mobile phone to the CPU of a computer. It is like the brain and the heart of the mobile phone.

The SIM card is in the form of a tiny chip which is actually a microprocessor that stores all the data and information of the mobile phone. The main feature of this chip is that it carries with itself an intrinsic telephone number. The user, who puts a particular SIM in his mobile phone, becomes the owner of that particular mobile phone number. If a SIM is taken from one instrument and put in another, then the second phone carries the number. Similarly, if a new SIM is put in the first phone, then that instrument answers to the call of the number carried in the new SIM.

The memory of a SIM card varies. Major Service providers like O2, Vodafone, Orange and T-Mobile have now started providing exciting features and bill plans in order to attract more and more customers. All mobile companies majorly offer two kinds of services: prepaid and post paid. Even consumers can now change their number, their service provider and the billing plan at the drop of a hat.

It is on the basis of the SIM card provided by mobile phone services providers that a bill is generated at the end of a billing cycle, and the consumer is charged according to the usage.