T-Mobile's "Device Unlock" app will unlock your phone, if you have this one model
Just to show you how this does work, let's pretend that you do own a Samsung Galaxy Avant (unless you do happen to own that model). You will have the option for a temporary or permanent unlock. The first option is for those going overseas for business or vacation and plan on using a GSM carrier in the country they are traveling to. If you select permanent, you will never be able to stuff the genie back into the bottle. In other words, once you unlock your phone permanently, you cannot lock it again.
Remember that your contract has to have been completed, and your phone cannot have been reported stolen for you to be able to unlock it. And while the app is quite limited now, eventually it should cover more models. You might want to check in every now and then to see if your phone has been added.
source: DeviceUnlock via RedmondPie