TracFone is a prepaid wireless company that operates a little differently than other major carriers. Instead of installing and using its own towers, TracFone buys airtime from other companies, then sells the minutes to its customers. Because TracFone is a little unusual in this way, its subscriber identification module cards can't be used with Global System for Mobile Communications carriers such as AT&T.
The GSM Network
The Global System for Mobile Communications network is the most widely-used technology for mobile phones in the world. Service providers who use the GSM network include AT&T, T-Mobile, Rogers Wireless and Vodafone. While these service providers have their differences, the technology they use in their phones is the same. This means that you can use any phone from any of the other companies. For instance a phone you bought from T-Mobile could be used with Rogers Wireless service. This is made possible in part due to a little device called a SIM card.
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A SIM card as is a small chip card that works in GSM phones. This chip card helps the phone communicate with its network, as well as store contact information, the phone's number and text messages. This makes it easy to transfer your phone information from one phone to another by simply removing the SIM card from one phone, and then inserting it into another. Most phones however, come locked to the network that you purchase them from, meaning that you can use only one service provider.
While GSM-enabled phones are designed to work with multiple service providers, the carriers don't want you to take advantage of this feature, so the phones come locked to their respective providers. To get one phone to work with a another service provider, you must first unlock it. This can be done by entering a special unlock code, which may be provided to you by your carrier (depending on its policies). After you insert a new SIM card into your phone, you enter your unlock code, and then you'll be allowed to use any SIM card from any carrier.
While some TracFone phones do come with SIM cards, these SIM cards aren't interchangeable with other GSM-enabled phones. This is because TracFone SIMs are programmed to recognize only TracFone phones. TracFone phones are created with firmware installed in them which syncs up with TracFone SIM cards upon activation. After activation, the phone won't accept any other SIM card, and the SIM card won't work in any other phone. The issue isn't simply between TracFone and AT&T. The real issue is that TracFone phones can only accept TracFone SIM cards.