These phones are not unlocked with codes, it is done remotely via the Apple servers, and there are two methods you can try.
First,insert a SIM card from a different network (make sure the SIM is not PIN protected).
Then start your phone and turn on WiFi. The phone should download a small software update, and then it will be unlocked - it will work on any network.
If that method isn't successful...
Still with the same new SIM, connect the phone to your computer and launch iTunes. The small software update should be downloaded and installed, and the phone should be unlocked.