Article: Insert the SIM card for the new network to check that the phone is locked. Many S3s are actually unlocked out of the box, so checking first can save you a lot of time. Your phone must be running Android 4.1.1 (It doesn't work with some Android 4.3 either) or later in order for this method to work. You can check your device’s version by opening Settings, and then scrolling to the bottom and selecting About Device. Look for Android Version to find your version number.  To update your phone, open the Settings and then scroll down to About Device. In the next menu, select System Updates and then Check for Updates. Your phone will attempt to update over the network automatically. Perform the network update over a wireless network, since your new SIM doesn't have any connectivity yet. You cannot unlock S3s that run on the CDMA network. In the US, this is Sprint. AT&T, Verizon, and T-Mobile all run on the GSM network, which allows you to SIM-unlock the phone. This method is not guaranteed to work with all versions of the S3, but you will do no damage in trying. You will need to enter a code in the dialer to open the Service menu. Once the dialer is open, enter the following code: *#197328640# This will open the maintenance main menu. If you select the incorrect option, press the Menu button on your phone and select Back. This will open the Debug menu. This opens a menu allowing you to adjust the settings of your S3. This controls the SIM lock function. After you select this option, wait about 30 seconds. This will return you to the NETWORK LOCK menu. Wait about one minute after selecting this option. After you have waited about a minute, reboot the phone. You won’t receive any confirmation that the process worked. If your phone connects to the new SIM card's network, then the unlock process worked.
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Check that the phone is locked. Update your phone. Ensure that your phone is a GSM phone. Open the dialer. Tap [1] UMTS. Tap [1] DEBUG SCREEN. Tap [8] PHONE CONTROL. Tap [6] NETWORK LOCK. Tap [3] PERSO SHA256 OFF. Press the Menu button and select Back. Tap [4] NW LOCK NV DATA INITIALLIZ. Reboot your phone.