What the app does is that it searches your phone for its SIM unlock code. Once the app finds it, you write it down and then place the SIM card you would like to use in the phone. When you are prompted for the unlock code, just enter the code that was found by the app and you should be good to go. However, like most apps that mess around with your phone and requires your phone to be rooted (a “freed” state where you have the ability to modify and access files you normally wouldn’t be able to) – there’s always a risk involved.
The app is called [root] Galaxy S2 SIM Unlock, and is available on the Android Market as a free download. Note: we have not tried it ourselves, but if you do, please leave a comment here.