A SIM lock, simlock, network lock or subsidy lock is a capability built into GSM phones by 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
- Mobile network code
- Mobile station identification number
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 (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.
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 one or two 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 or even sell the phone for a profit. SIM locking makes it more difficult to do this. Once your contract is expired, phone companies should be willing to give you your unlock code, however this can often prove difficult. Visit an online unlocking company who will charge you a small fee for this service.