iTure doesn't allow me to do so for the APPLE official version because it does some check against Apple, say my device can not be upgraded/restores. I have to use the custom jailbreak version to get it through. Apple tries controlling too much to all users. My next phone won't be iPhone if Apple still does all the trick.
See, AT&T nowdays will give you unlock code for all other phones after you use the phone for 3 months, expect iPhone because Apple controls that for all US customers. We have to use hackers code to in order to use other networks.