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A 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 with certain International Mobile Subscriber Identities (IMSIs); IMSIs may be restricted by: Mobile country code (MCC; e.g., will only work with SIM 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, some phones, especially 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 mobile 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; besides the locking, phones may also have firmware installed on them which is specific to the network provider. For example, a Vodafone or Telstra branded phone in Australia will display 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. Most phones can be unbranded by reflashing a different firmware version, a procedure recommended for advanced users only.
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 makes it more difficult to do this.