HI yes you can sure use your t-mobile in phone locked to t-mobile without any problem .Else if you want to use the t-mobile sim in the phone which is locked to some other network work then you first have to unlock it.You can easily unlock the phone using sites like Unlock-Free.com here they will generate unlock code for some older mobile models at free of cost .After unlocking the phone you can freely use it with any GSM sim .