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A SIM lock is a feature implemented by mobile phone manufacturers and providers to allow only the use of SIM cards in the phone that meet specific criteria. Currently, phones can be locked to only accept SIM cards from one or more of the following:
The country lock only allows the use of the phone with SIM cards that originated in a specific country or group of countries.
A network lock only allows the use of the phone with specific networks.
The most restrictive type of lock which can be used is the full SIM card lock, which means that a phone will only work with one SIM card. If that SIM card malfunctions or is damaged, the phone will no longer work and must be serviced.
Laws and policies on if and how phones are SIM-locked vary from country to country. In Finland, providers are not allowed to use any type of SIM locking, though they are not unique: many other countries also prohibit locking, but many more only allow locking for the duration of the customer's service contract, after which, the provider must make provisions to unlock the customer's mobile. Notably, providers such as AT&T Wireless will never unlock handsets, ever after a customer has fulfilled their contract with them.
At the other end of the scale is Hungary, where almost all phones are SIM-locked for life and unlocking them is prohibited.
The code required to remove all SIM locks from a phone is called the master code or network code key. In the past, it was generated using a mathematical formula, usually based on the phone's unique IMEI or MCC codes. However, this became too easy to break, and, nowadays, the unlock codes are generated randomly and retained by the provider.
Typically, a locked phone will display a message if a restricted SIM is used, requesting the unlock code. In some cases, the phone will simply display a message explaining that it is locked. This is especially the case with handsets provided by AT&T Wireless.
For example, on the Sony Ericsson T610 mobile phone, "Insert correct SIM card" will appear on the phone's LCD display if the wrong SIM is used. Once a valid unlocking code is entered, the phone will display "Network unlocked".
One of the most popular ways phones are unlocked is using the RS-232 or LPT port of a computer using software usually written specifically for the model of phone being unlocked. In some cases, special "phone clips" are used which re-program the software that controls the phone, removing the SIM lock. However, such clips are usually very expensive.
Unlocking a phone without the permission or unlocking code from the provider is usually in breach of the agreement with the provider, though most countries do not make specific laws prohibiting the removal of SIM locks. For example, in Poland, the law states that providers cannot word their customer contracts so that they forbid the removal of SIM locks, and the process is entirely legal providing that the IMEI number of the phone is not changed during the unlocking process.
In the United Kingdom, there are also no specific laws that prohibit unlocking, however, under the Mobile Telephones (Re-programming) Act, changing the IMEI of the phone is illegal.
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