You can put a
sim card from t-mobile into any t-mobile phone, and any "unlocked" phone that uses a
sim card, only problem you run into sometimes, is if you used a phone unlocked lets say from at&t you may need to have it "flashed" so that you can access "t-zones" because it will try and fail to access the internet programed to that phone from its provider. Also, some phones require certain plan add-ons, like blackberry, sidekick,etc...