Unlock SIM card

Mobile phone manufacturers make GSM phones with a SIM lock, a built-in capability to restrict the use of these phones to specific countries and network providers. In this way, phones can be locked to only accept SIM cards from a specific country, network or service provider, or only a particular SIM type. In fact, most mobile phones are shipped with country and network provider locks these days.
Unlock SIM card. It is now either mandatory or voluntary for the provider to unlock the SIM card of the customer. Still, some providers are reluctant on doing it even after a customer has fulfilled their service contract. A few others will unlock SIM card of a customer only if he/she has an active account in good standing for at least 90 days. Some network providers charge a fee for releasing the unlock code.
Unlocking laws vary from country to country. For example, in Belgium, networks are not allowed to sell locked handsets. In The Netherlands, providers must provide unlocking codes, but are permitted to charge a fee for this during the first 12 months after purchase. The Hong Kong government does not allow networks to sell locked mobile phones, though the restriction does not apply to some PDA phone models, such as those offered by BlackBerry. In the US, unlocking is now legal.
Unlock SIM card and special codes. You can unlock a handset by entering a special code, or in some cases, over-the-air by the carrier. In most cases, a locked phone will display a message if a restricted SIM is used, asking for the unlock code. The code needed for removing all SIM locks from a phone is called the master code or network code key. The phone itself verifies the unlock code, which is either stored in a database or calculated with the help of a complex mathematical formula supplied by the provider.