In response to device unlocking being made legal in the U.S., T-Mobile has released Device Unlock for Android, which allows the carrier's customers to unlock their Android devices. Currently the app only allows Samsung Galaxy Avant users to unlock their handsets, which means a vast majority of T-Mobile customers will not be able to make use of the app at this time. Hopefully the carrier will expand unlocking support in the coming months.
You can download the app directly on your Samsung Galaxy Avant through the Google Play Store or by using the link which follows:
If you own a Galaxy Avant, you will have the option to temporarily or permanently unlock your device. The temporary unlock can be employed by customers who plan to travel out of the country and use their smartphone on an international GSM carrier. The permanent unlock will ensure your device is fully unlocked, and it cannot be locked to T-Mobile 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 should check back later to see if your device is unlocked.