A network lock, known by many names including SIM lock, simlock, or subsidy lock, is a technology that wireless communications carriers use to prevent the mobile phones they sell from being used on another network. Cell phones can be locked to a particular country, network, or subscriber identity module (SIM) card. A “locked” phone is usually sold to customers at a discounted price, since the carrier expects to earn revenue from the customer by providing wireless service to the phone. Customers wishing to “unlock” their phone can request the carrier provide them with an unlock code or may turn to third-party services or tools.
There are several reasons a mobile phone might not work on a particular wireless carrier’s network, ranging from incompatible technologies to deliberate lockouts. Most wireless networks around the world use a standard known as the Global System for Mobile Communications (GSM); in a few countries, however, other standards are sometimes used. In the U.S., for example, some carriers use GSM while others do not. GSM networks can also operate on several different radio frequencies, and not all GSM phones support every frequency used globally. Many phones also include a network lock which limits the phone’s portability between carriers.
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A network lock is generally built into a phone by the manufacturer, and can come in several forms, depending on the phone, region, and carrier. For the most part, a phone will only work with a SIM card from one provider. It is also possible to lock a phone to an individual SIM card or to SIM cards from a particular country, though these methods are less common. Generally only GSM phones include such a lock, as the underlying technology of other types of networks keeps consumers from switching carriers.
Wireless carriers frequently include a network lock on cell phones they sell to consumers to prevent them from switching to a competitor’s network. Locked phones are often sold at a heavily discounted price or given away to attract new customers. The carrier expects to earn back the cost of the phone by being the sole provider of service. A customer who purchases a locked phone may also be compelled to sign a long-term contract.
In many cases, a network lock can be removed from a phone through a process known as “unlocking.” Consumers may wish to unlock their phones for many reasons, including a lack of reception on a particular network, a desire to avoid expensive international roaming fees, and the availability of cheaper plans from other carriers. Under certain circumstances, some carriers will provide customers with a special code to unlock the phone. Alternatively, there are tools and services tailored to phone unlocking, many of which are available online. In most countries, it is legal to unlock a phone for use on another network, but consumers should always double check their local laws before taking any action.