Then you have to go through third party unlock services to get it unlocked or get the unlock code. I'm not sure if AT&T unlock it from their side.
There are many online unlock sites available. Following are some
The problem is you have to pay for the unlock code. I'm afraid your phone is backlisted and the unlock might not work.
Re Pantech, did you try these steps ?
Can you try these key combination on your Pantech ?
Turn off the phone completely. Then hold down Volume down and Power button at the same until a reset menu appears ? If it boot it back to lock screen, then release the buttons.
Turn it off. Then hold down Volume Up, Volume Down and Power for several seconds and does it boot into any menu ?
Or did you try the default combinations like 0000,1234, 1122 ?