that's imposible! cdma phone don't have a simcard and t-mo is a gsm that have a simcard. if you unlock your phone you can activate that to cdma network like sprint and verizon but not on t-mo, at&t, cingular.
the phone that you mentioned, 3585i, does not support GSM, it is built for CDMA networks, and therefore cannot be used on any GSM carrier, and t-mo will give you a better phone on sign-up anyway, why would you want to keep that 3585i?