A Key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phonebook. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries. Subscribers can purchase a variety of unlocked cell phones which come directly from the manufactuer with out any types of lock on them so they can be used with any carrier world wide. The only consideration should be to check the frequency range used in the country the cell phone will be used. A cell phone with all four frequencies (850 900 1800 1900mhz) will work with any GSM carrier.
In Australia, Canada, Europe and the United States many operators lock the cell phones they sell. This is done because the price of the cell phone is typically subsidised with revenue from subscriptions, and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card). It is always possible to switch to another (non-locked). such handsets can be purchased from various dealer online who deal in unlocked cell phones.
Some providers will unlock the phone for free if the customer has held an account for a certain time period. Third party unlocking services exist that are often quicker and lower cost than that of the operator. In most countries, removing the lock is legal. United States-based AT&T and T-Mobile provide free unlocking services to their customers. customer can call in and find out about the time required.
unlocked cell phones are avaiable around the world thanks to the internet as well as manufactuers setting up their own flagship stores to sell these phones. A person can purchase a phone in one country and use prepaid sim cards to utilized his phone in any part of the world.