When you buy the phone from a carrier, you pay a subsidized rate for that shiny new phone, but in reality, you are paying a high price by signing a contract with that carrier. Your phone is locked to that carrier, which means you cannot use a SIM card from another network.
After some time you may think of switching to a different carrier, maybe to get more advantage from another carrier. You may also think of switching because the services provided by your current provider are not satisfactory. In such a case, you can unlock your phone. Since the phone is locked by a software code, there's a good chance to unlock it. If you own a Galaxy Note 2, or a Galaxy S3, here's how you can unlock your Android smartphone.
This method comes courtesy of XDA Developers, who have found a hidden menu in Samsung's own software. This method has been tested on N7105 and N7100 for the Note 2 and some Galaxy S3 variants.
Disclaimer: ValueWalk will not be held responsible for any damage done to the phone. Follow the process at your own risk.
This method only works on devices that are carrier locked. Please check if your phone is locked and then follow the below tutorial.
Before proceeding, note that your phone should be running on Jelly Bean 4.1 otherwise some of the menu choices won't work.
Step 1: On your device, dial the following keys: *#197328640#
Step 2: From the main menu, navigate to  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options.
Step 3: From Options, go to Perso SHA256 OFF. Wait for about 30 seconds and then go back one step by pressing the Menu button and then go back once. You should now be back in  Network Lock.
Step 4: Choose  NW Lock NV Data INITIALLIZ. Wait for one minute.
Step 5: After a minute, reboot your device.
Your phone is now carrier unlocked and you can use any carrier on your phone. You won't get any sort of confirmation on the screen, but don't worry, the phone is unlocked. Try inserting a SIM from a different carrier to test it out.
If you are stuck somewhere and need help, then check out the XDA Developer forum from below.
(Source: XDA-Developers forum)