Editing SIM lock (section)

== Unlocking technology ==
A handset can be unlocked by [[Subsidy Password|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 Programmable|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''. 
If the phone is network locked it will typically display one of the following messages: SIM network PIN blocked, Enter lock PIN.

There can also be multiple levels of locks placed on the phone by networks, which block the use of other networks' SIM cards. These are usually referred to as "Network Control Key" (NCK) and "Service Provider Control Key" (SPCK), additionally, a Regional lock exists which is specific to Europe and it is called "Region Control Key" (RGCK).

Typically, a locked handset will display a message if a restricted SIM is used, requesting the unlock code. On recent phone models running Android software, the phone will display a message saying "SIM network unlock PIN" or "Enter Network Lock Control Key" if network locked. Windows phones will display the message, "This SIM card can only be used on specific networks. Contact your customer service center for 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. Once a valid code is entered, the handset will display "Network unlocked" or "Network unlock successful".

The unlock code is verified by the handset and is generated by the manufacturer, typically by an algorithm such as a one way [[hash function|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. Other manufacturers have taken a more cautious approach, and embed a [[random]] number in the handset's firmware that is  retained by the manufacturer and the network on whose behalf the lock was applied. 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]].

=== Hardlocked phones ===
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".

=== Economics ===
Handset manufacturers have economic incentives both to strengthen SIM lock security (which placates network providers and enables exclusivity deals) and to weaken it (broadening a handset's appeal to customers who are not interested in the service provider that offers it). 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 ([[Prepaid mobile phone|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 (phone)|Debranding]] involves reflashing or replacing the firmware to remove the operator logo or any limitations or customizations 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 the 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 telephone calls|prepaid subscription]].

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 a 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.

=== Box breaking ===
{{Main|Box breaker}} 
A practice known as ''box breaking'' is common<ref>{{cite web|url=http://www.mobilenewscwp.co.uk/2011/06/20/box-breaking-investigation-exposes-retail-staff-breaches/|title=Box breaking investigation exposes retail staff breaches|work=Mobile News Online|access-date=7 October 2014}}</ref> 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.<ref>{{cite web|url=https://www.theregister.co.uk/2003/07/28/fraud_cases_up_financial_losses/|title=Fraud cases up, financial losses down|website=[[The Register]]|access-date=7 October 2014}}</ref><ref>{{cite web |url=http://www.tax-news.com/asp/story/story_rss.asp?storyname=24716 |title=HMRC Stings Missing Traders for GBP12 Million |website=www.tax-news.com |access-date=13 January 2022 |archive-url=https://web.archive.org/web/20110716194943/http://www.tax-news.com/asp/story/story_rss.asp?storyname=24716 |archive-date=16 July 2011 |url-status=usurped}}</ref>

=== Unlocking services ===
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 website. 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. Some companies also offer unlocking services that require sending the handset's IMEI number. Other companies sell unlocking hardware, including devices which fit between the SIM card and the phone to spoof the original network identifier during registration and devices to read and edit the handset's firmware. 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.

==== Unlock code generators ====
There are online services that will provide an IMEI unlocking service for [[List of Nokia products|DCT4 and DCT3 Nokia]] mobile phones.{{citation needed|date=May 2013<!-- do not use promotional web sites as references - see [[Wikipedia:Reliable Sources]] and [[Wikipedia:Spam]] -->}} This method of unlock requires the user to know which carrier the mobile phone is locked to, and also needs to provide an IMEI. 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.

For [[List of Nokia products|DCT4 and DCT3 Nokia]], unlock codes consist of a "#" key, followed by "pw+", 10 (DCT3) or 15 (DCT4) digits, "+", and another number ranging from 1-7, and finally ends with a "#". Depending on the carrier which the phone is locked to, only some codes will work with the mobile phone. Most phones respond to the unlock codes ending in +1# or +7#, however some phones are configured to allow only one of the seven codes to work.  The following is an example of a DCT4/DCT3 unlock code:
<pre>
# pw+931882753035021+7#
</pre>

DCT4 and DCT3 Nokia will only allow a maximum of five unlock attempts using the unlock code method. 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.