Most of us experienced issues with locked devices on specific networks, which is a real pain in the *** , because usually requires some experience using hex editors, or even paying your carrier or a third-party to unlock your SIM.
A senior member on XDA forums by its nickname Grooby has put together a nifty Windows application (tool) that will ease the otherwise difficult method created by XDA recognized developer Odia for people who are having trouble to manually unlock a device.
Network/Sim Unlock Code Generator works ONLY for older models of the Galaxy S II I9100, as this method does not work with hardware version 1.4 or above. The older version of the tool will also work for users carrying a Galaxy S I9000.
* Added GPU supported/not supported message
Note: Be very careful when inserting the unlock code you only have 3 chances to insert the correct code after that the phone locks itself. If you already inserted it once or twice without success stop trying and contact project developer to assist you.
Download Version (1.1-GPU) With UI (as on screenshot)
Download Console Version (1.0-GPU) (outdated, only for SGS1,use UI version!)