Many of us must be aware of the concept of unlocked cell phones. These are getting more and more common, and an increasing number of people are using them. With these, one gets the freedom to use a particular mobile phone on any network. To get a clearer idea, one first needs to understand what a locked cell phone is. It is a phone that is tied or 'locked' to a specific service provider, and can only be used on the network provided by them. On the other hand, an unlocked cell phone is not tied or 'locked' to any service provider and can be used with different service providers worldwide. Locked mobile phones only work with the Subscriber Identification Module (SIM) card supplied by the service provider to which the cell phone is locked. One can use different SIM cards in the locked phone, as long as they belong to the service provider that has provided the cell. If it is unlocked later, then it can recognize and work with any SIM card, irrespective of the service provider. An example would help you better understand: 'AT&T' and 'Sprint' are two major service providers. A cell phone 'locked' to 'AT&T' or who's SIM card is provided by 'AT&T', will only work with 'AT&T' SIM cards. SIM cards by other providers, such as 'Sprint' , 'Verizon', etc, will not work on the phone. But if the phone provided by 'AT&T' is unlocked, it can be used it with other SIM cards provided by 'Sprint' or any other service provider. Similarly, cell phones provided by 'Sprint' can't be used with 'AT&T' SIM cards or any other service provider for that matter, until they are unlocked. Service providers usually enter into a contract with the manufacturers, and get a particular system incorporated in cell phones, which disables the use of other networks on their handsets. In the US, the Apple iPhone works only on the AT&T network. One cannot use the iPhone with other service providers, such as Sprint or Verizon. But once unlocked, it can be used with SIM cards that belong to any service provider. If one needs a feature-rich mobile phone that is tied up in a contract with a service provider and is not available with other networks, he may need to purchase it with a plan, and unlock it later. This is considered an option for people who need the same locked cell phone, but with a different service provider. However, as most providers have a 2 year contract with early termination clauses, this can be a very expensive option. Mobile phones that work on the CDMA networks cannot be unlocked. However, locked cell phones working on the GSM networks can be unlocked. There are also disadvantages of using unlocked phones. If the service provider offers some exclusive facilities such as 3G and GPS on their high-end cell phones, these features would not work on an unlocked phone, or with another service provider. In some cases, service providers help unlock the phone if the contract period is successfully completed, or if traveling out of the country. Unlocking a mobile phone privately, without informing the respective service provider, may be illegal in some countries. However, mobile phone manufacturers may also offer unlocked cell phone for sale on their website or company outlet. If they do, you surely have a good chance to buy the phone you've been thinking of unlocking.