A:Answer Originally, GSM cellphones were restricted to the specific branded cellphone network that provided the phone. Cellphone-network companies did this to ensure recovery of their subsidies for phones they provided at deep discount to their contract-plan consumers. For example, if you contracted with Cingular, then your phone would only work with a Cingular SIM card. "Unlocked" means that a GSM cellphone can work with a SIM card for any GSM cellphone network. When you buy a cellphone outright, you are entitled to have it unlocked. If you have fully paid for a network-restricted T-Mobile phone, T-Mobile will unlock it for you. The only rare exception is if the phone itself was specifically designed to only work with one network. For example, T-Mobile was unable to unlock my very old Sharp TM150. New or Used is irrelevant. Cheers!