you can use it with your existing T-mobile sim card to unlock Iphone 3g..It works fine but the problem is u wont be able to use Itunes, caz sometimes it says that the "inserted sim is not supported" that is the only problem. Also, If you are lucky you would be able to import your contacts from the sim card caz sometimes that features doesn't work either and you find other way around to do that. like Google contact, Neusync, and funambol..etc. but other than that it works all fine....one more thing is that if you are not getting signals then it would say " no sim card" then just turn ur iphone off and then turn it on ...it will be fine...