Many network providers lock phones to their network and sell them at low cost. When users buy such phones they need to stay with that network till the contract ends. In this way the network provider increases their potential. Incase a user needs to unlock their phone in the middle of contract they can request their respective network provider. If they deny there are many online unlocking vendors like Mobile-unlocker.com who provide permanent unlock through their remote unlocking service.You can get your iPhone 3G unlocked using their service.