If you're not a big fan of SIM-locked phones, there's a surefire way to freedom. Purchasing an official unlock key can definitely solve your problems, but if you're short on cash, or you're just not willing to part with your money, have no fear. Nothing is impossible for the Android development community.

XDA Developers member spocky12 has compiled the GalaxSim Unlock app, which allows you to do away with that annoying SIM lock on your Samsung Galaxy S3 GT-I9300. The developer, however, warns that the app may not keep your phone SIM-unlocked forever, as Samsung may inject checks in future ROMs that may relock the phone.

This guide shows you how to SIM unlock the Galaxy S3 I9300.

Warning

• The instructions in this guide are intended for use with the Samsung Galaxy S3, model number GT-I9300. Applying these instructions on another device or model may produce undesired outcomes.
• The information in this guide is provided for instructional and educational purposes only. There is no guarantee that these instructions will work under your specific and unique circumstances.
• Use these instructions at your own risk. We shall not hold any responsibility or liability for whatever happens to you or your device arising from your use of the info in this guide.
• Read and understand the whole guide first before actually performing the instructions.

Requirements

• A rooted Samsung Galaxy S3 GT-I9300, with BusyBox installed.
  • Check out our guides on:
    • How to root the Galaxy S3 GT-I9300 via Odin or you can watch the video if you are having trouble visualizing it.
    • How to root, install ClockworkMod Recovery and other hacks/mods using the Samsung BS3 Toolkit v2.0.
• Enable Unknown Sources on your Galaxy S3 under Settings > Security.
• Install a file manager app on your Galaxy S3, such as ES File Explorer, from the Google Play Store.
• Download the GalaxSim APK file (GalaxSimUnlock.apk, 115 kB) to your computer.
• Make sure your device's battery still has at least 70% power. You don't want the device to shut down in the middle of the procedure.
• Backup all personal data on your phone to make sure you have a copy of your personal data (e.g., contacts, SMS, MMS, Internet settings, Wi-Fi passwords, and the like) in case the procedure in this guide erases such data.
  • For backup tips, check our guides on how to sync your data to the cloud and how to create local backups of your mobile data.

Instructions

1. Copy the GalaxSim Unlock APK file to your Galaxy S3′s SD card.
2. Launch the file manager app on your device.
3. Locate the GalaxSim APK file and tap on it to install it on your Galaxy S3
4. After installation is complete, launch the GalaxSim Unlock app on your device.
5. Grant it root access by pressing the Grant/Allow button.
6. Tap the Backup EFS button to backup your EFS/IMEI data.
   • Your EFS/IMEI data will be backed up in the GalaxSimUnlock folder on your internal storage. Do not delete this folder. We also suggest backing your EFS/IMEI data to a safe place on your computer.
7. Tap the Unlock button to unlock your Galaxy S3.
8. Reboot the phone.

Congratulations! You can now use any carrier's SIM card on your Samsung Galaxy S3 GT-I9300.