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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. In the United Kingdom under the Mobile Telephones (Re-programming) Act 2002, changing the IMEI of the phone is illegal. In the Netherlands unlocking is legal provided that the process does not overwrite the handset's flash memory with a (modified) copy of copyrighted firmware, since this would be a breach of copyright retained by the manufacturer. However, unlocking a handset will void its warranty.

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 Cingular Wireless.

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. The algorithms used in almost all Nokia brand phones (based on IMEI and MCC code) have been reverse engineered, stolen or leaked, resulting in many people offering nokia unlock codes for free or for a fee. Many other manufacturers have taken a more cautious approach, and embed a random number in the handset's firmware that is only retained by the network on whose behalf the lock was applied.

Handset manufacturers have economic incentives both to strengthen simlock security (which placates network providers and enables exclusivity deals), but also to weaken it (broadening a handset's appeal to customers who are not interested in the service provider that offers it). Also, making it too difficult to unlock a handset makes it less appealing to network service providers that have a legal obligation to provide unlock codes for every handset they've ever sold.

The main reason to unlock a phone is to be able to use it with a different SIM card. For example, when travelling abroad it's usually cheaper to temporarily use a foreign network. In some cases, a simlocked handset is sold at a substantially lower price than an unlocked one, because the service provider expects income through its service. A consumer may choose to unlock the phone and continue using his previous provider. Therefore, simlocks are usually employed on cheaper (pay-as-you-go) handsets, while discounts on more expensive handsets require a subscription that provides guaranteed cash flow.

Unlocking via computer

One of the most popular ways phones are unlocked is using the RS-232 or LPT port of a computer using software usually written specifically for the model of phone being unlocked. In some cases, special "phone clips" are used which re-program the software that controls the phone, removing the SIM lock. However, such clips are usually very expensive.