Mobile phones use a card called a SIM card. This stands for Subscriber Information Module, and each one contains a unique serial number. The SIM card identifies the person who is using the phone. The mobile phone companies lock the phones so that a SIM card from another company can not be used. While most phones can be easily unlocked from your own home simply tracing a cell phone number by downloading the proper software and the right code, Samsung unlocking is a bit more complicated and must be done in personal by qualified techs. There are quite a number of websites that sell the codes to unlock the phones.