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cell phones SIM lock, simlock, network lock or subsidy lock is a capability built into GSM phones by mobile phone manufacturers. Network providers use this capability to restrict the use of these phones to specific countries and network providers. Generally, phones can be locked to accept only SIM cards based on the International Mobile Subscriber Identity, which has elements of:

Mobile country code (MCC; e.g., will only work with SIMs issued in one country)
Mobile network code (MNC; e.g., AT&T Mobility, T-Mobile, Vodafone, Bell Mobility etc.)
Mobile station identification number (MSIN; i.e., only one SIM can be used with the phone)
Additionally, at least Nokia phones can lock group IDs which are used in voice group call service.

In most countries, most mobile phones are shipped with country and/or network provider locks.

Most cell phones can be unlocked to work with any GSM, such as O2 or Orange (in the UK), but the phone may still display the original branding and may not support features of the new carrier. Most cell phones can be unbranded by uploading a different firmware version, a procedure recommended for advanced users only.

Locked phones may also have firmware installed on them which is specific to the network provider. For example, if you have a Vodafone or Telstra branded phone in Australia, it displays the relevant logo and may only support features provided by that network (e.g. Vodafone Live!). This firmware is installed by the service provider and is separate from the locking mechanism.

Unlocking technology
A handset can be unlocked by entering a special code, or in some cases, over-the-air by the carrier. Usually the unlock process is permanent. One example where it is not is the Apple iPhone, which is officially unlocked (when applicable) every time during its activation step. The code required to remove all SIM locks from a phone is referred to as the master code, network code key, multilock code.

Typically, a locked phone will display a message if a restricted SIM is used, requesting the unlock code. For example, on the Sony Ericsson T610 mobile phone, "Insert correct SIM card" will appear on the phone's display if the wrong SIM is used. Other phones may display a "Enter special code" or "Enter unlocking code". Once a valid unlocking code is entered, the phone will display "Network unlocked". In some cases, the phone will simply display a message explaining that it is locked.

The unlock code is verified by the phone itself, and this code is calculated by the network provider, typically by a complex mathematical algorithm that involves the IMEI of the phone in question. The algorithms used in earlier Nokia brand phones (based on IMEI and MCC code) have been reverse-engineered, stolen or leaked, resulting in many people offering Nokia unlock codes for free or for a fee.

Many other manufacturers have taken a more cautious approach, and embed a random number in the handset's firmware that is only retained by the network on whose behalf the lock was applied. Such phones can often still be unlocked, but need to be connected to special unlocking boxes, such as UFS or JAF and the Universal Box that will rewrite that part of its firmware where the lock status is kept, and often even recover a phone that is "bricked" or completely damaged in the software sense. Common characteristics of such phones include no response from the phone on attempting to switch it on, though in some cases it can be recovered or "unbricked" merely by holding some keys down while switching on the phone.

Most cell phones have security measures built into their software that prevent users from entering the unlock code too many times. After that the phone becomes "hard-locked" and a special unlocking box (mentioned above) has to be used in order to unlock it.

However, certain cell phones, notably the high-end PDA's and PocketPC's, are unlocked by the use of special programs that require the phone to be connected to the computer and read the unlock code. So, such cell phones can be unlocked free of cost.

Handset manufacturers have economic incentives both to strengthen SIM lock security (which placates network providers and enables exclusivity deals), and also 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.

The main reason to unlock a phone is to be able to use it with a different SIM card. For example, when traveling abroad it's usually cheaper to temporarily use a foreign network, for example with a prepaid subscription. An unlocked phone can't access extra cell phone towers or give free phone service. All it can do is accept other SIMs.

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. Consumers may choose to unlock their phone and continue using their previous provider. Therefore, SIM locks are usually employed on cheaper (pay-as-you-go) handsets, while discounts on more expensive handsets require a subscription that provides guaranteed income.

Also, the unlocked phones have a far higher market value, even more if they are debranded. Debranding too involves special unlocking boxes that remove the operator logo on startup and a variety of limitations that have been imposed on the device by the operator to increase income, as in the case of the iPhone in which the mobile operator AT&T disabled a feature called "tethering" that allows the phone to be used as a modem on the computer.

A practice known as box breaking is common in the UK and some other markets. This involves purchasing (usually) pay-as-you-go handsets 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 subsidized handset is then either thrown away or sold or used elsewhere. This practice is entirely 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" may be legal, most "box breaking" businesses are doing illegal things like importing/exporting box-broken phones to other countries (to sell as grey market goods) without paying import duties, defrauding various tax authorities on VAT frauds and substituting counterfeit batteries or chargers. Recently network operators have been insisting that new customers purchase substantial amounts of airtime at the same time as they buy a new handset[citation needed], in order that the total price they pay comes close to the true value of the handset.

Unlocking via code
Some companies offer an email unlocking service. This service requires that the individual who wishes to unlock a phone email their IMEI number, which is usually displayed by any phone upon entering *#06#, to the company. The company will process this IMEI number and email back an unlock code and instructions. Input the unlock code and the phone is unlocked. These email services are usually the most efficient as it is the same method most retail stores will offer. However there are several fake random number generators available, so you must be careful. Some vendors also offer unlocking by means of physically mailing the cell phones to a technician.

Spoofing SIM data
In 2004, a company BLADOX in the Czech Republic released a small device called a Turbo SIM which contained a small MCU capable of spoofing the network ID during SIM registration. This thin device sits between the SIM card and the cell phones, in the SIM slot. A small piece of plastic on the SIM is removed to make room for the MCU. The most popular of these devices, originally for the U.S. Apple iPhone 3G, is typically called "Universal Sim" after a label printed on the card.

Unlocking on-the-fly via software
To work around the iPhone's SIM lock system, a popular method used to unlock is to install a third-party software application on the iPhone which overrides the carrier lock temporarily.