Unlocking a SIM is quite handy especially when you travel a lot and you want to get rid of the carrier restrictions so that other SIM cards could also work on your device. Though unlocking a SIM isn’t required for rooting or flashing your ROMs but only required if you want to use any other network on your carrier phone. So any of you who got a Samsung Galaxy S III can now unlock SIM and experience other networks on their smartphone.
The method is quite easy and comes in a .zip file that contains some files that are needed to move to the Galaxy S III EFS folder. This method is written by XDA Recognized Developer zohawkish, and that is how he describes the method:
-First you need to backup your “EFS FILE” in safe place.
– you must be in unlocked stock firmware like BLG8 (using voodoo app + libsec-ril.so (mike1986 fix))
– unzip the attached zip “efs.zip”
– copy the files (.nv_core.bak, .nv_core.bak.md5) to your EFS file
– reboot and enjoy !!!
To keep your data safe and if things go wrong in some place therefore it is advised to make a backup of your EFS file. And as expected, Jelly Bean update is coming soon to Galaxy S III so in order to update to Jelly Bean you will be needed to restore the EFS file, so either way it is better if you make a backup before proceeding further. Users have reported that this method for SIM unlock works perfectly for them and shouldn’t be a problem for anyone.
Source: XDA Developers