Well my ATT Wireless branded smt5600 hard locks at the Windows Mobile Screen. I've done an IMEI unlock from the c500 reset website where you get the .unl file twice, and I've also hard reset it twice and I get nothing, it still locks at the windows mobile screen.
I have no ATT branded sim, but I have one on the way. I'm using both a cingular and tmobile sim. Could it be that its the victim of a failed upgrade or unlock attempt? Is it permalocked?
I've done at least 10 searches and I've got nothing helpful.