GENERAL: What is, and where is, my SIM card?
Your SIM card is used to store various items of information related to your GSM phone's operation. It is used to identify your network operator and subscriber identity - effectively your phone number, although your actual phone number is not stored on the SIM. It also contains storage space for your contacts list and some sms messages. Without a SIM card inside of a GSM phone, the phone will not operate (apart from making emergency calls.) In most cases, you can insert your SIM card into any compatible phone, and that phone will become 'yours'.
There are two types of SIM card - full-sized and small-sized SIMs. The full-sized SIM is the same size as a credit card, while the small SIM (the most common) is a cut-down version about the size of a thumb nail. Both have a set of metallic contacts on one side, in a standard position, which are used to communicate with the on-board microprocessor. Most GSM phones have them easily accessible once the battery pack has been removed. If you have difficulty locating it then please refer to your phone’s instruction manual.
Please ensure that you replace your SIM card correctly back into your phone, else the phone will not work.