In most places, the iPhone is only allowed with carriers (cellphone service providers) that Apple has an agreement with. Someone who buys an iPhone also signs a contract with the carrier, which is an agreement to pay for their services. A mobile phone contract usually lasts for one or two years, and the person buying the phone pays the carrier every month. At first, they pay less than the iPhone is worth because they will pay the rest later through fees as part of their contract. The contract also says the iPhone may only be used on their network. However, some people do not like that, so they unlock their iPhone so it can be used with other carriers.