Many Android users wish to unlock their phones in order to use them with local SIM cards while traveling, or on domestic carriers that don't offer an Android phone for sale. Fortunately, this is easily accomplished with an unlock code and a second SIM card. Be sure to do your research before attempting to unlock your phone, since unlocking can cause unintended consequences such as decreases in data speed.
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Obtain an unlock code. T-Mobile users can wait three months from the date their contract starts and then request a free unlock code from T-Mobile. If you're not a T-Mobile customer or wish to unlock your phone immediately, purchase an unlock code online. You'll need your IMEI number, which you can find by going to Settings > About Phone > Status.
Turn off your Android phone and remove the back cover and battery. Look for the SIM card, and remove the one that's currently in your phone. Set it aside. Insert the new SIM card and replace the battery and back cover. Turn your phone back on.
When the phone boots up, it will display a screen asking for an unlock code for the new SIM. Enter the unlock code when prompted. Your phone is now unlocked.