Generally this is a sign that the iPhone had previously been
hacked/modified/jailbroken and the restore relocked it to the
original wireless carrier. If this is the case, only that wireless
carrier can unlock your iPhone. You must contact them to see
if they offer unlocking and if you qualify.
Where did you acquire this iPhone?
What wireless carrier did you use before this problem?
Does the app Cydia appear on your iPhone?
What does it say when you look at Settings=>General=>About=>Carrier?