Conventional cell phone service providers works as follows:
Each mobile phone has a unique serial number, for GSM Phones it is a “IMEI #,” which is used by the cell phone network provider to to identify who you are. It is like a unique finger print for your cell phone. The wireless service provider ‘knows’ that a particular serial number belongs to your phone and therefore knows where to send your phone calls when someone dials your phone number.
A GSM phone and network service is different because it is not directly linked to your cell phone. It is linked to a chip called a subscriber information module (SIM) card that is placed in your cell phone. The SIM card behaves the same way as the serial number but it is not unique to one phone. Instead, the SIM has the unique serial number.
The important difference is that you can put your SIM Card into any compatible GSM phone and that phone will now act as your phone. This is because the wireless network provider sees your SIM Card not the phone that it is inside. A GSM network provider does not care that you have changed phones, all it cares is where to find your SIM Card.
The reverse is true for your GSM mobile phone. Not only can you put your SIM Card into any compatible GSM phone, but you can also put any compatible SIM Card into your GSM phone. What this means is that you can use the same cell phone with different SIM Cards from different networks around the world.
Any GSM phone can work with any SIM. Therefore, if you have already purchased an expensive phone that you like and have learned how to use it, if you change from one GSM wireless provider to another compatible GSM Company, you do not have to change phones. You can keep the phone you have grown to love, and all you need to do is replace the first network’s SIM with your new service provider’s SIM Card.
This is where unlocking is useful. Even though your GSM mobile phone was originally designed to work with any SIM card, the network provider locks the phones they sell to ensure that you can only use the phone on their network so that they can keep your business for themselves. This limits the use of your phone on their network only. Unlocking your mobile phone enables you to use the cell phone you love on any network, anywhere around the world.