Most GSM Mobile Stations (MS) are sold to be used on a specific GSM network. The Mobile Stations are “locked” to only accept a Subscriber Identity Module (SIM) from the GSM carrier that sold the Mobile Station.
There are several situations where you would want to unlock your Mobile Station. For example, you might wish to switch GSM service providers and keep your current Mobile Station. Or, you may have purchased a new Mobile Station and you want to sell your old Mobile Station to someone who is subscribed to a different GSM service provider. A third situation is if you are travelling extensively in another GSM service providers area and you would like to get local GSM service instead of paying roaming charges.
There are two methods of unlocking Mobile Stations. The first method, which requires no equipment, is to enter the unlock code for your Mobile Station from the keypad. Your GSM service provide will be able to give you this unlock code, but may not be willing to do so. The second method is to use dedicated unlocking hardware which plugs into your Mobile Station and unlocks it. You may purchase this hardware, or send your Mobile Station to an unlocking service which will unlock it for a small fee.