Useful facts about the iPhone SIM card
Fact 3. The iPhone's SIM card can be locked via a SIM lock.
In certain countries, such as the US, the iPhone 4's SIM card can be locked via a SIM lock. The SIM lock associates the phone with your carrier to ensure that it only operates on that carrier's network. Usually, a SIM lock is put in place when a company subsidizes the price of the phone and, therefore, wants to make sure that subscribers honor their subscriber agreement for a certain time frame. There are several ways to remove a SIM lock; however, you should read your contract with your mobile operator to avoid doing anything illegal.