iphone4 02 network free??
 

is Iphone 4 of 02 network free? i just updated my iphone to OS5 and activated it with the original sim card which is 02. in the process of activation a message came up that "CONGRATULATION YOUR PHONE IS UNLOCKED" to my greatest surprise the phone begin to work with every other sim cards. I tried the same steps with iphone 3 network operator but it could not unlock.
My question is this, what is the secret behind the O2?