HTC one X sim unlock issues

[tmoney3581]

I have an at&t HTC one X that was supposed to be unlocked when I got it and when I put in my non at&t sim the sim unlock screen doesn't come up which leads me to belive it is. The problem is when I search for available networks the only thing that shows up is at&t and US which leads me to believe it's not unlocked. How can I unlock it if the screen to enter the unlock code doesn't come up when I put my sim in? Is there another way such as a code to punch in on the dialpad that would bring this screen up? Any help with this issue would be greatly appreciated

 

[IAmSixNine]

If you put in a non att sim card and you werent asked for an unlock code then it is Unlocked.
Give us more info.
What sim? what carrier? location might help also.
Who is US? USCellular? i always thought they were CDMA.. and didnt they get bought up by Verizon? i could be wrong with that..