If you have a locked Samsung Galaxy S3 or Note 2, you can unlock it for free. This enables you to use it worldwide, and with any SIM card you fancy.
How to unlock Samsung Galaxy S3 & Galaxy Note 2
So, say you live in the US and you have a phone with AT&T – you can only use it with AT&T. Using this method, you can free yourself from this restriction. You could use your Samsung Galaxy S3 or Note 2 with any US carrier you wanted to – or indeed any other carrier anywhere in the world. You could send the phone as a gift to an overseas relative, then they could use it with their favourite carrier.
Here’s how you do it:
Insert the foreign SIM card into the Samsung Galaxy S3, then turn it off and on again with the SIM card in.
The Note or Samsung N7100 can be unlocked, along with many other phones that are still being tested. Go into the dial pad and type in *#197328640#
Go to Main Menu >  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options Perso SHA256 OFF > and then wait at least 30 seconds. Go back a step by pressing Menu, select Back and you should see  Network Lock, then choose  NW Lock NV Data INITIALLIZ
To ensure that you didn’t make a mistake reading, and we didn’t make a typo, and to ensure that the computer displayed things properly, the sequence of characters should be: Asterisk Pound One Nine Seven Three Two Eight Six Four Zero Pound. Some people call the asterisk button the star button, and call the pound button the hash button simply because of the symbols they carry.
Hopefully, you’ll now have an unlocked GS3 or Note 2, so the phone can travel the world and use any SIM.
This code has been used by Samsung for a while now, so it should work on other phones as well.