iPhone 5 LTE problem?

I recently purchased a new iPhone 5 from one of my buddy's. I put my SIM card in and it worked fine. I had good service from T-Mobile, but I noticed that the top no longer said LTE only E. I used the phone for a while thinking it might change and it didn't. So I took my old phone, and put my SIM card back into it, and it had LTE? any Ideas?