T-Mobile uses GSM phones. They work with a sim chip inserted to activate the phone. Their store phones are locked. You can search for used ones that have been unlocked. You can buy new or used unlocked versions of any GSM phone direct from the manufacturer or several online stores. Even Amazon.Com sells a lot of them. The phone will be more expensive unlocked. Quad band GSM phones work best with at&t service. Some configuration is required to access the web etc... at&t doesn't activate phone they use sim cards now called smart chips try reading this http://danmobile.blogspot.com/2007/04/cdma-vs-gsm.html then check out Dan Mobile's blog right column for an extensive list of links/bookmarks including sites that sell unlocked phones http://danmobile.blogspot.com/