Usually, when you purchase a mobile phone, the company you have bought it from will have “locked” it so that you can only use it on their network. This means that if you try to use a SIM from another carrier in your handset, the phone will not work. The reason for this is that mobile networks subsidise mobile phones- margins on calls and texts are enormous, but the network makes back the subsidy from your future top ups. Locking the phone prevents you from taking advantage of this subsidy and then switching to a new network. You might see this lock referred to as a SIM lock, a network lock, or a subsidy lock.
Mobile phones can be unlocked by entering a special code. Some carriers will give you this code if you ask for it and have been a customer for a long time. This is the safest way to remove the lock, but not all carriers will do this, and many people try to source free codes from the internet. Reasons might include if you were travelling to another country and wanted to use pre-paid SIM cards to avoid large roaming costs, or if you want to use a newer phone with an existing SIM.
As well as the lock, the network also installs a firmware that “brands” the phone, so that it displays the relevant logo and only supports the features provided by that particular network. While many users find it easy to unlock phones, firmware is harder to remove, so even an unlocked phone may not support all the features of the new carrier, and might still display the original branding.
It is not illegal, but there are risks involved whenever you try to unlock your phone. Some networks might void your warranty if you remove the lock. Many phones only allow you to try a certain number of unlock codes- Nokia, for example, has a limit of five tries. After that, the phone will become “hard-locked,” which means that it cannot be unlocked unless special equipment is used. This is one reason why it is better to ask your carrier for a code rather than relying on free codes found on the internet. Many newer phones are manufactured with stronger protection, so they won’t respond to codes generated by the free unlock programs.
The Apple iPhone has been particularly difficult for users to unlock. The first few publically available unlocks involved specialised knowledge and equipment, which resulted in many users destroying their phones in the process. Even safer methods require quite a complicated procedure, and come with the danger of causing the phone to “brick”, meaning you won’t be able to use even your original SIM card. Apple has issued an official warning to discourage users from installing unauthorized unlocking programs.