After the contract period, you may request your network provider to unlock your phone, but it's not as easy to unlock as you think. S., unlocking became illegal after January 26th, 2013.However, a new bill is in progress to make it legal again.Before start unlocking, make sure you have properly backed up your device.
If your phone didn't ask for Unlock Code, it's permanently unlocked to use with any GSM carrier.
You may have to downgrade your firmware version in order to let this method work.
alternatively you can call your service provider and ask them for SIM unlock code without paying from your wallet.
Why unlock your Samsung Galaxy S3 to use with a different SIM card?
One possible reason is that you love your phone, but not your cellular provider because of the poor service or an unreliable connection.
If you purchased a Samsung GS3 with AT&T, T-Mobile, etc.on a contract, it might be locked for a specific amount of time called the contract period.Your phone must be running on Android 4.1.1 or later to successfully use this method.Check your device Android version by opening the Settings menu, scroll to the bottom, and select "About Device" and check your phone's "Android version" number.If your phone is not updated to Android 4.1.1 or higher, you can do it by selecting the next menu to "About Device," select "System Updates," and then click on "Check for Updates." Your phone will automatically looked for the update over the network.Insert the non-accepted SIM Card and power on the Phone.