Perhaps you have heard of someone finding a great deal on a cellular phone on EBay or Craigslist, and had it "unlocked" so they could use it on their own carrier account. That phone was almost certainly not a Sprint cell phone or a Verizon cell phone. Why? It is due to the technology behind those phones versus that of a cell phone from a provider like AT&T or T-Mobile use.
Sprint and Verizon use what is known as Code Division Multiple Access (CDMA) technology, whereas many other providers use Global System for Mobile Communications (GSM) technology in their phones. In 2007, GSM phones made up about 80% of market share, obviously the more popular choice among carriers. The difference is in the technology itself.
CDMA phones rely on the network of service provider to authenticate the user's phone and signal, therefore these are not able to be easily manipulated by the owner of the cell phone. To accomplish this process in a GSM type phone, there is a Subscriber Identity Module (SIM) card installed in the phone. This tiny, removable card contains the contacts, settings and phone information on it. If the card were removed from one person's phone and installed into another phone, it would allow that individual to use the second phone as their own, provided that phone were unlocked. A CDMA phone does not have a SIM card and is considered permanently locked to the user.
Throughout the world, most phones are the GSM type. The United States is the exception to this generalization, although there are carriers that use GSM technology. This is beneficial to people traveling abroad because if they have an unlocked phone, they are able to utilize the service in other countries rather than being forced to pay international rates from their home carrier network, which can be very expensive. The more common reason for unlock cell phone is to either use a preferred phone on a new different network, or to allow an upgraded phone to be used on an existing account that is not near its term end.
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