It is not possible to use your Boost Mobile SIM card in a T-mobile device (locked or unlocked). In order to understand the reason for this, you must first understand what the SIM card is designed to do. The majority of people know that SIM cards are very important for storing their contact lists and personal information, but those little cards also have another purpose. They are used to store important network information. Hence the name "SIM", coming from Subscriber Information Module. Also, they can be used to store information that helps drive certain aspects and features of the device (i.e. Camera, PTT, MP3 playback, etc). With the current set of information stored on your card, you can only use it in phones designed to work on a TDMA network. From my experience, you could theoretically use your SIM card in a Nextel Network handset, but not a T-mobile device (T-mobile uses a GSM network). And even then, it would not be guaranteed that your phone would be fully functioning (or functioning at all for that matter). The general touchpoints to remember when attempting to place a SIM card for one carrier into a device designed to operate on another carrier are: *Both the SIM and phone must originate from similar networks (i.e. GSM to GSM, TDMA to TDMA, CDMA to CDMA) *Usually, the device that is receiving the SIM has to be "UNLOCKED" in order to operate on the SIM's network, or you may risk damage to both the SIM and phone *It doesn't hurt to do some research on the web to see if the specific SIM/phone match you are attempting has been done by someone else with any sort of success