This could be nothing more than a grammatical error, but in the off chance this was a slip of the tongue, I'm posting the conversation I had with a T-Mobile Agent on Live Chat. Originally the conversation was about improving the coverage in my area, but after that issue was (kinda) resolved, I started asking other questions while I was on the chat. Here's part of the copy of the chat.
The rep said "THE upcoming Windows Phone device." Like I said, this could be nothing, so take this with a grain of salt, but I just found it very interesting. While I was on the chat she also said that the 2520 is not planned to be carried on T-Mobile, but you can use the other carriers' devices on their network with their free 200MB monthly plan as long as you buy and activate a SIM card for it.You: Okay, thanks. Could you answer one more question of mine?
Analyn C: No problem.
Analyn C: Sure.
You: Do you know if any new Windows Phone 8 devices are coming to T-Mobile?
Analyn C: I will check that for you.
Analyn C: Right now, we don't have any updates for the upcoming Windows Phone device.
Analyn C: You can check our website from time to time, Joseph.