Dear,

if you have updated with iTunes, your firmware version is 3.1.3, your baseband or modem firmware is 5.12.0. then your stuck....
if your firmware is 3.1.3 and baseband or modem firmware 5.11.-- then you still have chance to unlock...this case let me know ill send you the link

you can check the above from..Setting>General>About

Hope your phone is software unlocked, not official unlock

hope the above is informative..