The term Unlock code is by Mobile phone Manufacturer to refer to the 8 or 16 digit unlock code that is necessary to remove the service provider lock (also known as subsidy lock) from mobile phones.
The term "subsidy" refers to the subsidization of cell phones sold to customers that purchase mobile phones as part of a contract with the mobile service operator. Many network operators provide these subsidies as an incentive for users to sign extended contracts, and use the subsidy lock as a means of protecting their investment.
The installation of such locks means that users often are forced to purchase a different phone if they decide to switch their network (of advantage to both the mobile phone manufacturer and cellular service provider).
A SIM lock, simlock, network lock or subsidy lock — not to be confused with PIN or PUK code
is a capability built-in to GSM phones by mobile phone manufacturers. Network providers use this capability to restrict the use of these phones to specific countries and network providers. Currently, phones can be locked to accept only SIM cards from one or more of the following:
Countries (the phone will work in one country, but not another)
Network/Service providers (e.g. AT&T Mobility, T-Mobile, Vodafone, etc.)
SIM types (i.e. only specific SIM cards can be used with the phone).
In some countries, most mobile phones are shipped with country and/or network provider locks.
In addition, these locked phones tend to 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 (i.e. Vodafone Live!). This firmware is installed by the service provider and is separate from the locking mechanism.
A handset can be unlocked by entering a special code, or in some cases, over-the-air by the carrier. 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. 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. This is especially the case with handsets provided by AT&T Mobility.
The code required to remove all SIM locks from a phone is called the master code or network code key.
The unlock code is verified by the phone itself, and is either stored in a database or calculated using an obscure mathematical formula by the provider.
SIM cards are small smart cards that fit inside phones based on the GSM technology. SIMs contain personalized information about its user including the network activation and even phone book entries. One can put its SIM card in another GSM phone and use it as if it was his own phone.
Service offered by most cellular service providers, allowing users to use their phone even if they are not in their home coverage area. In general, using a phone while &roaming& incurs additional fees, though some service providers offer national roaming for free on some service plans. When a phone is in a &roaming area&, it usually displays an indicator light or logo such as &R& or &ROAM&. An increasing number of phones offer what is called Internation or Global Roaming, since it allows users to make and receive phone calls from virtually any country.