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It has a removable account card, called a SIM (subscriber information module) and this SIM has a unique serial number on it.
There are three reasons why you might want to unlock your phone: 1.
Because you want to change the wireless company you get service from in the US, and want to keep your present phone. You've changed to a different service provider and got a new phone, and you want to unlock your old phone, so it can be use in case of an emergency, by swapping the SIM if your new phone ever has problems. Because you want to travel internationally, and don't want to have to pay expensive international rates your wireless company would charge you for international roaming.
When traveling internationally, GSM service is offered in 207 countries.
There are hacking instructions online to unlock many phones, but a phone that is unlocked improperly can end up non-functional.
Carriers operating on the GSM network use SIM cards.