From The Apple Wiki

Network Control Key or NCK is a 15-digit key required to "legitimately" unlock an iPhone. The mechanism to unlock the phone with a valid NCK is still present and usable in X-Gold 608 and presumably later basebands. It is, however, superseded by the policies set in the WildcardTicket that is generated during the activation process. An unlocked phone receives a wildcard ticket that has no restrictions set.