A SIM card is the device (a small plastic card about the size of a thumb nail) that enables your phone to work and provide a phone service. The SIM card stores information that identifies the phone to the network service provider and thereby allows the phone to connect to the network and make calls. In many phones the SIM card can be removed and replaced with a different SIM card.
A different SIM card will work in your existing phone if your phone is a GSM compatible phone and is not "locked" to your network provider.
Many network operators sell "locked" phones. "Locked" means that the phone is restricted by the network operator to work only on its’ own or affiliate networks. If your existing phone is locked, you cannot use a SIM card supplied by another network provider without unlocking your phone first.
Check if your phone is locked - Your phone may be locked if you purchased it on contract or pay as you go. To check, insert a SIM card from another network into your phone and switch it on. If you are able to view the menu then your phone is unlocked. If the phone does not allow you to view the menu, but displays an error message such as "Phone restricted", "SIM not valid", or "SIM card not accepted, insert correct SIM card", your phone is most likely locked to your network provider. If you are not sure, contact your provider and ask them if your mobile phone is locked or not.
In most cases it is a simple process to unlock your phone. You have several options: