Pantech Matrix C740

I wasn't aware that you could request a certain phone from a warranty replacement. Are there certain restrictions or circumstances? I have the Matrix and I downloaded an app from AT&T's website and now I can't access the Applications folder. The phone goes to a white AT&T screen with a progress bar at the bottom, then says SIM CHECKING, then returns back to the main screen. I've tried rebooting it and doing a hard reset, nothing seems to work. Had my eye on the Xenon.