Using the Controls on the Top of Your iPhone
You'll find several important controls along the top of your iPhone. They include the headset jack, the SIM card tray, and the Sleep/Wake button.
Sleep/Wake button: This button is used to lock or unlock your iPhone and to turn your iPhone on or off. When your iPhone is locked, you can still receive calls and text messages, but nothing happens if you touch its screen. When your iPhone is turned off, all incoming calls go directly to voicemail.
SIM card tray: The SIM card tray is where you remove or replace the SIM card inside your iPhone.
A SIM (Subscriber Identity Module) card is a removable smart card used to identify mobile phones. It allows users to change phones by moving the SIM card from one phone to another.
Headset jack: The headset jack lets you plug in the included iPhone headset, which looks a lot like white iPod earbuds. Unlike the iPod earbuds, however, the iPhone headset has a microphone so that you can use it to talk as well as listen.
The headset jack is recessed, so most third-party earphones (such as those made by Shure, Etymotic, and Future Sonics) don’t work with it. Fortunately, for around $11, you can buy an adapter from companies such as Belkin that enables you to use just about any brand or style of earphones you like with your iPhone. Fortunately, Apple listened to customers. The iPhone 3G doesn’t have a recessed headset jack and doesn’t require an adapter.