KU990 calling feature behaves erratically.

I have a 2-year contract with my provider, AT&T. My KU990 has a T-Mobile interface that displays with a greeting tone when I turn the phone on. Fifty percent of the time the phone will switch from saying "AT&T" to "Emergency calls only," in which case I can't make calls. This will even happen when I'm in the same room not subjecting the phone to any movement. (I purchased the phone new, as it was advertised as "unlocked" for AT&T and numerous other providers. AT&T registered me a new sim card, but that didn't solve the problem.) Very irritating! Any remedies? Thanks. - mok951@yahoo.com