T-Mobile knows that with all these devices coming out left and right some of the best ways for customers to obtain said products is trading in their own mobile devices. T-Mobile starting on September 17th wants to give you the best bang for your buck and will match any trade in from a competitor such as AT&T, Verizon, or Sprint. Better yet they will give you a cool $50 on top of that. The $50 credit you will see within 2-3 bill cycles.
The offer starts on September 17th and you can read more about it here