Why is it that some phones needs to be unlocked?

[Constantine]

Some of my friends bought phones that requires unlocking. Could anyone tell me why manufacturers of this phones require this step? How does it work?

[grexia]

To stop customers from leaving for another company so they block the phones for exclusive activation on their network. As far as I know, those who were able to purchased this kind of phone were not able to use their phones to other networks. So if I were you, avoid buying them. I never had a phone which needs to be unlocked so I can't tell you how it works, I guess the information is kept confidential.