if you buy a phone from any service provider (at&t, Cingular, t-mobile) the phone is lock to their network.
gsm phone have a
sim card (small card like comp chip) let say you have t-mobile phone and you want to use a cingular
sim card, your. t-mobile phone will not gonna take that cingular
sim card because that phone is lock to t-mobile. it only take t-mobile
sim card. but if you have unlock phone! that phone will take any kind of
sim card. if you are inside the u.s it take at&t, cingular, t-mobile if you are in canada, it will take fido or roger. it will take any kind of
sim card there is in the world. that what good about unlock phone. just check it out ebay to get the idea. just search for "unlock cell phone" and all the unlock cell phone will be in front of you. take care now.