The reason many network providers SIM lock their phones is that they offer phones at a discount to customers in exchange for a contract to pay for the use of the network for a specified time period, usually between one and three years. This business model allows the company to recoup the cost of the phone over the life of the contract. Such discounts are worth up to several hundred US dollars. If the phones were not locked, users might sign a contract with one company, get the discounted phone, then stop paying the monthly bill thus breaking the contract and start using the phone on another network or even sell the phone for a profit.

SIM locking is very common if subsidized phones are sold with prepaid contracts. It is important to note, however, that the technology associated with the phone must be compatible with the technology being used by the network carrier. In other countries, carriers may not be required to unlock devices or may require the consumer to pay a fee for unlocking.


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Unlocking the phone, however, is almost universally legal. A handset can be unlocked by entering a code provided by the network operator. Alternative mechanisms include software running on the handset or a computer attached to the handset, hardware devices that connect to the handset or over-the-air by the carrier. Usually the unlock process is permanent. The code required to remove all locks from a phone is referred to as the master code , network code key , or multilock code. These locks can be removed using the corresponding unlock codes, which are unique to each phone depending on its IMEI.

Typically, a locked handset will display a message if a restricted SIM is used, requesting the unlock code. Other handsets may display different messages such as "Enter special code" or "Enter unlocking code," or in some cases the handset will simply display a message stating that it is locked. The unlock code is verified by the handset and is generated by the manufacturer, typically by an algorithm such as a one way hash or trapdoor function.

Sometimes big telecom providers change the original factory unlock codes as an extra layer of security against unlocking services. For various big brands such as Samsung and Motorola there is no algorithm but just a random code generator where the unlock codes are programmed in the phone itself and then saved in a big database managed by the manufacturer.

For the other brands where the unlock codes are still based on algorithms those are based on the IMEI number and the MCC code and have been reverse-engineered , stolen or leaked. Some handsets can be unlocked using software that generates an unlock code from an IMEI number and country and operator details using the algorithm specific to the handset. These handsets can still be unlocked by online services that have access to either inside people with the manufacturer or with the telecom networks, or they need to be connected to the computer with a cable where specific software will bypass the security and SIM-unlock the phone.

Sometimes this is done by advanced calculations to bypass the security the official way and other times using exploits or overwriting parts of the firmware where the lock status is kept, and often even recover a phone that is bricked or completely damaged in the software sense. Most handsets have security measures built into their firmware that protects them from repeated attempts to guess the unlock code. After entering more than a certain number of incorrect codes the phone becomes frozen. This is a state where the phone will display a security message that the phone needs a service.

Older phones could no longer be used at all at this point, however modern smartphones often keep working with the original SIM but require extra work to then unlock them correctly. In extreme situations physical access to internal hardware via in-circuit debugging may be utilised for example, via JTAG headers on a circuit board. Such access may be required to modify initialization software used for booting. A hardlocked phone is one in which a telecom provider has modified the firmware on the phone to make it impossible to manually enter the unlock codes in any way.

The only solution to SIM-unlock such a phone is to change the firmware to a firmware which has not been modified by any telecom provider, a so-called "unbranded firmware". Also, making it too difficult to unlock a handset might make it less appealing to network service providers who have a legal obligation to provide unlock codes for certain handsets or in certain countries.

In some cases, a SIM-locked handset is sold at a substantially lower price than an unlocked one, because the service provider expects income through its service. SIM locks are employed on cheaper pay-as-you-go handsets, while discounts on more expensive handsets require a subscription that provides guaranteed income. Unlocked handsets have a higher market value, even more so if they are debranded.

Debranding involves reflashing or replacing the firmware to remove the operator logo or any limitations or customisations that have been imposed on the handset by the operator, and is usually accomplished with software designed for a particular handset model, however most smart-phones can be debranded and unlocked solely with use of special software.

The main reason to unlock a handset is to be able to use it with a different SIM card. Consumers may wish to continue using their previous provider with a new handset or when traveling abroad they may wish to connect to a foreign network with a prepaid subscription.

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Nevertheless, the fundamental principle of GSM and its successors, is open interfaces which encourage competition among multiple vendors. This is the reason a mobile phone is, in fact, a combination of phone and the subscriber identity module SIM. Locking the phone to a network is not much different from having the SIM built into the mobile phone. Network operators in many industrialized countries are not bound by law to give the phone unlocking code to subscribers even after the expiry of the contract period. Mobile phones with multiple SIM cards are quite common in India.

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A practice known as box breaking is common [11] in the United Kingdom and other markets. This involves purchasing subsidized handsets usually pay-as-you-go from retail stores, unlocking the phones, and then selling them often abroad for a higher price than the subsidised retail price. The SIM card that came with the handset is then either thrown away, sold or used elsewhere. This practice is legal in the UK and provides a de facto limit to the extent to which networks are willing to subsidize pay-as-you-go handsets.

While the act of box breaking is legal, some businesses are also engaging in illegal activities such as exporting the box-broken phones to other countries, to sell as grey market goods without paying import duties known as Carousel Fraud or substituting counterfeit batteries and chargers. Some companies offer an online unlocking service. This service requires that individuals who wish to unlock a handset provide their IMEI number and sometimes also country and operator details to the company, either via email or a web site.

The company will then provide the unlock code for the handset. For some brands such as Nokia and Samsung various services also offer special remote-unlocking software with instructions, where a cable is needed to remove the SIM lock at home. Such companies may email the unlocking code or software which will remotely unlock the device. The pricing for unlocking a device will vary depending on the network it is locked to and the handset model itself, as each unlock code is unique to each individual handset. Generally, older model Nokia unlock codes are free and instantly retrievable by these services.

The unlock codes retrieved must be entered into the mobile phone using the keypad. Depending on the carrier which the phone is locked to, only some codes will work with the mobile phone. After five incorrect codes have been inputted, the phone will not allow the user to try any more codes even if it is correct and will require the owner to try other unlock methods. Many countries listed below have some form of SIM-locking laws specifying the period of SIM locking and the cost of obtaining unlocking codes.

As there is no competition, consumers usually buy standard mobile phones that are not locked to any specific carrier. In Austria , unlocking is allowed at any time by the owner of the device. T-Mobile Austria charges euro to unlock the iPhone for prepaid subscribers and in contract subscribers.

For subscribers who have finished their 2-year iPhone contract, T-Mobile Austria charges 50 euro to unlock the iPhone. There does not appear to be any regulation or law on SIM locking in Australia. One law professor, Dale Clapperton, gave a talk stating that bundling iPhone and mobile phone service could be violating the Trade Practices Act. In Brazil , SIM locks are not prohibited. However, the mobile carrier must inform the consumer of the existence of a SIM lock.

After this regulation most telecom operators started voluntarily unlocking the devices as soon as it was purchased so one could leave the store with an unlocked phone. Under revisions to the Canadian Radio-television and Telecommunications Commission CRTC Wireless Code of Conduct effective 1 December , all new devices must be sold unlocked, and carriers must offer to unlock phones purchased prior to this date free of charge.

After the implementation of this rule, Bell Canada initially refused to offer unlocks for users who were not customers of the carrier in contrast to Rogers and Telus , but reversed course in February due to public backlash. Under the original version of the Wireless Code implemented , carriers were required to offer unlocks no later than 90 calendar days from the start of a contract for subsidized devices, or immediately upon purchase of an unsubsidized device.

The Code, however, did not expressly prohibit carriers from charging an unlock fee. The act would not have banned SIM locking but would have required wireless carriers to unlock phones at no charge at the end of a cell phone contract. The bill was introduced in two sessions of parliament but failed to pass either time. Under a regulation enacted by the Ministry of Industry and Information Technology, locking phones to a specific carrier is prohibited if other carriers are also using the same type of network technology.

Therefore, all phones approved to be sold in China are never locked to begin with regardless of whether the consumer purchased the phone under a contract or not. However, since all three Chinese carriers each uses a different network technology after the adoption of 3G, carriers started to ask phone manufacturers to disable support for network technologies not used by such carrier even if the phone has been originally designed to be capable of supporting those network technologies. Such a move does not result in violation of the ban on phone locking.

Starting October 1, , all the mobile telephone services providers, must sell to all users unlocked devices and provide free of charge support to unlock previously sold devices. This regulation was ordered to enable mobile number portability and to facilitate the reduction on costs ordered simultaneously. Since Chile 1 January , newly sold phones must be unlocked.

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Previously bought locked phones had to be unlocked for free. The regulation was put in place in order to implement mobile number portability. It does not cover international unlocking for use outside Chile, so users may have to pay for the unlocking service. A new related issue is present since 23 September , every IMEI have to be registered so the phone works in the country. For local carriers, they do the process, but to use a phone from outside the country, each user has to register it. IMEI Registration. Such request can be made immediately after buying the phone, and the operator has a day period to fulfill the request.

For devices bought on a prepaid plan, the user has to wait at least 12 months before submitting such request. Many of the carriers choose not to lock the phones. Only Hi3G "3" lock their phones, but can only do so for six months. After six months, the carrier is obliged by law to unlock the phone free of charge. But the consumer needs to contact the original supplier, and provide the IMEI and original phone number for which the phone was sold.