Originally Posted by THE TECH
Yeah, that's how it was done in the past but with Apple making it harder and harder I wasn't sure if it was as easy as that anymore. Also, since the iPhone 4 uses a micro sim card, I wasn't sure if A. T-mobile was using them as well and B. a Gevey type sim chip was also needed or not.
AH okay. Well I did it with my girlfriends iPhone because she has t-mobile as well(before t-mobile starting unlocking phones for free). I gave her my old phone when I upgraded. You just need to trim the t-mobile sim card slightly on the plastic part, and it will fit in the micro-sim tray. Make sure not to cut it too much or then it can move around the tray. I just measured the micro sim to hers and cut it
something like this: