That's definitely strange - if it's worked before, it should still work. I'm wondering if there's something that got a little off in your device that's caused it to forget that it's T-Mobile phone. Let's try a master reset to see if that will help get things back to normal. Here's a link to the steps: http://support.t-mobile.com/docs/DOC-4276#Master_reset
Will you try this and see if it helps?