Advantages
An unlocked phone will not be permanently tied to just one network, and it can be used with any GSM service provider. If you want to keep your phone but move to a different service provider, you can do that with an unlocked phone. After unlocking, simply store the device when you do not need it, and use it as a backup when your main phone has problems. Another advantage is when you travel abroad, you can use other GSM operators with your unlocked phone without paying the usually higher roaming rates that your own network may charge.
Compatibility
An unlocked phone works with compatible GSM operators in more than 200 countries, depending on frequency. GSM providers operate in different frequencies, usually 850 MHz, 900 MHz, 1,800 MHz and 1,900 MHz. All GSM phones are built to support one or more of these available frequencies, and after unlocking, the phone will work with all networks that operate on its frequency band.
SIM
A SIM card is a small, flat rectangular card that fits into a predefined slot inside your GSM phone. Cellular service providers who operate on GSM technology identify subscribers solely based on the SIM. Since GSM service providers do not physically associate a mobile phone with a subscriber, you can change your mobile phone without any consequences. As long as you use the SIM, you will be identified regardless of mobile phone.
IMEI
One common way to unlock a GSM phone is to feed a secret code to the interface. Phones from manufacturers, such as Nokia, have a unique serial number called the IMEI (International Mobile Equipment Identity) that is associated with only one device. A GSM network operator uses this unique IMEI to generate a secret code for the particular phone. You can unlock your phone by navigating through the interface (which depends on phone's manufacturer and model), and entering the code when prompted.
Firmware Reinstall
Another way to unlock a phone is to remove the device's existing firmware and install a new one. Firmware is the operating system that a hardware component, such as a mobile phone, runs on. The firmware includes instructions that tell how the hardware and software are to interact. Most firmware are proprietary, meaning that it cannot be replaced or upgraded from any source. Phones from manufacturers such as Motorola, Siemens and Sony Ericsson can be unlocked by installing new firmware.
