- The iPhone 3G is only available from AT&T, and like all of AT&T's mobile devices, it uses Global Systems for Mobile Communication (GSM) technology, which requires a small, plastic chip known as a SIM card. Inserting an AT&T SIM card into the iPhone 3G links the phone with your AT&T account and all of the features associated with it, such as your phone number, service plan and upgrades, including Internet service. The presence or absence of a SIM card in the iPhone 3G does not affect its wireless adapter.
- The iPhone 3G's wireless adapter is not enabled by default, so if you use the device without a SIM card and you are having trouble connecting to a wireless network, it's likely that your adapter is switched off. To turn on the wireless adapter, tap the "Settings" button on your iPhone 3G and then tap "Wi-Fi." Tap the grey "Off" button, which will change to a blue "On" button. Look over the list of Wi-Fi connections that appears and tap the one you wish to use. Enter your password, if the iPhone 3G prompts you to do so, and tap "Join."
- Using an iPhone 3G with AT&T service gives you access to AT&T's 3G service, a telecommunications network that allows you to make clear calls and quickly send text and multimedia messages, as well as surf the Web at high speed. If you don't have a SIM card inserted in the iPhone 3G, your speed will depend on that of the Wi-Fi network you are using. Speed can vary greatly from one Wi-Fi network to another.
- Removing the SIM card from your iPhone 3G does not absolve you from any financial obligation you might have to AT&T. For example, if you have one year left on your mobile contract and you decide to use the iPhone 3G without a SIM card, you will still be billed for your monthly service or, alternately, you can pay AT&T's early termination fee as spelled out in your contract, even though you aren't actively using your plan. Don't remove the SIM card from your phone unless you have already canceled your contract.