A SIM lock, 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 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)
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 Nokia or ZTE (in Nigeria), 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, if you have a MTN or Visafone branded phone in Nigeria, it displays the relevant logo and may only support features provided by that network (e.g. MTNOnline!). 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 thousands of Naira. 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.