The GSM (Global System for Mobile Communications) network is the most widely used wireless technology for mobile phones in the world. Service providers in Asia, the Americas and Europe use GSM technology. Popular GSM service providers in the United States include AT&T and T-Mobile. Phones that are purchased from GSM companies include SIM cards. These cards store your contact information, text messages, calendar and e-mails. You can remove a SIM card from one GSM phone, and insert it into another, using a process called unlocking. However, you can't insert the SIM card into a non-GSM service provider.
SIM cards are small, thin cards that hold information, and are used in a plethora of different cell phones, including AT&T's iPhone. While SIM cards may be removed from one phone, and put into a phone belonging to another provider, it can't be done with all phones. There's a distinct difference between the wireless technology used by different service providers, and part of that difference involves SIM cards.
The other major wireless technology for mobile phones in the United States is the CDMA network. Popular service providers who use CDMA technology include Verizon and Sprint/Nextel. The main difference standard users will notice between CDMA and GSM technology is that CDMA phones don't have SIM cards. Instead, the information is stored directly to the phone itself. Because of this, the phone you get is locked to the service provider who sold it to you. You can't use it with any other service providers, unlike GSM phones, which allow you to use alternative carriers' SIM cards.
The process by which one GSM phone's SIM card can be inserted into another GSM phone is called unlocking. All phones with SIM cards in them are built to be used with multiple service providers (as long as they use GSM technology). But because network providers don't necessarily want you using a different network, they lock the phones before they sell them to you. By obtaining a unique unlock code (which is sometimes free; depending on the network), you can enable any GSM phone to be compatible with any GSM SIM card. But again, not all networks use GSM technology. Spring/Nextel, Verizon, MetroPCS and Boost Mobile all use CDMA technology, which means you cannot use SIM cards with those networks.
Although you're not able to use any SIM card in any network, there are many networks that do allow SIM card swapping. Many of these are in foreign countries, which will save you a good amount of money on long distance calling, because you can use local SIM cards, and be charged local rates. However, SIM cards won't work in any CDMA-enabled phones, such as the ones purchased from Verizon or Sprint/Nextel. The alternative to swapping SIM cards is called porting. This will transfer all of your old phone's information into your new phone. It's not as simple as unlocking, which means that you'll need a professional to do it, but it gets the job done. By porting, you'll also get to keep your old phone number, and you can even port from GSM phones to CDMA phones. Your old SIM card will be worthless, but all of the information that's stored on it will be transferred to your new CDMA phone.
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