If you purchased your iPhone from AT&T, it uses a SIM card to transmit information to and from your carrier. In some instances, the SIM card also stores phone numbers but beyond this, it has a limited function. Although it's possible to use your AT&T in another phone under certain circumstances, doing so will not enable any of the iPhone's functions on a different device.
When you purchase an iPhone from AT&T, the phone comes "locked" to AT&T -- but the SIM card itself can be used in any other device from AT&T. Like iPhones, most other mobile devices are sold locked to their respective networks, so if you want to use your iPhone's SIM card in a non-iPhone device you didn't buy from AT&T, use a device that you or someone else has "unlocked" to work on any network.
How SIM Card Transfers Work
When you insert your SIM card into a compatible device, whether it's another AT&T phone or an unlocked one from another carrier, your phone will connect with the AT&T network and within several seconds, allow you to make phone calls and send messages, from the same phone number you were using on your iPhone. As you click through your Contacts menu, you'll notice that some, but perhaps not all, of your iPhone contacts appear, depending on whether you earlier set your iPhone to save contacts to your SIM card.
Since SIM cards function only to transmit signals to and from cell phone carriers and store contacts' phone numbers, the most you can expect when you insert your iPhone SIM card into a different device is that you'll be able to make and receive phone calls using your old phone number. Nothing about an iPhone's SIM card is different from the SIM card in any non-smartphone, so inserting an iPhone's SIM card into a phone incapable of accessing the Internet won't make it suddenly capable of getting online.
Replacing Your iPhone
If at any point, you decide to replace your iPhone, the same rules for inserting your iPhone's SIM card into non-iPhone devices apply -- re-insert the SIM card from your iPhone, or any AT&T SIM card, into your new iPhone. Doing so gives you access to your contacts -- and to AT&T's network, via your original phone number. This doesn't work if you purchase your new iPhone from Verizon Wireless, whose devices use a technology that doesn't accept SIM cards.
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