Samsung Galaxy S/SGH-T959V/SIM Unlocking
SIM unlocking a device allows a phone to use the SIM cards from other carriers. This is also known as carrier unlocking.
You can try calling T-mobile and asking them to unlock it for you, but sometimes that is not an option or they refuse to do so.
If that is the case, you can follow the guide found here to do so, but you must be rooted.
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