I was recently overseas and had my GSM T-Mobile phone with me. I figured I would run out get a pre-paid sim card and have local cell service while in the country I was in. To my surprise I could not get the SIM card to work in the phone. I was a bit pissed. But to my surprise a friend of mine sent me a link to this article and now I have the unlock code to my cell phone and when I am overseas again I will get a local pre-paid SIM card and be in business. [Popular Science]