Stereo Headset

The headset included with iPhone features a microphone and an integrated button that allows you to answer and end calls easily, and control audio and video playback. The headset included with iPhone 3GS (shown here) also has volume buttons.

Plug in the headset to listen to music or make a phone call. Callers hear you through the built-in microphone. Press the center button to control music playback and answer or end calls, even when iPhone is locked. (You can also use the equivalent button on your Bluetooth headset to control these functions.)

Pause a song or video:  Press the center button once. Press again to resume playback.

Skip to the next song:  Press the center button twice quickly.

Return to previous song:  Press the center button three times quickly.

Fast-forward: Press the center button twice quickly and hold.

Rewind:  Press the center button three-times quickly and hold.

Adjust the volume (iPhone 3GS only): Press the + or – button.

Answer an incoming call:8 Press the center button once.

End the current call: Press the center button once.

Decline an incoming call:  Press and hold the center button for about two seconds, then let go. Two low beeps confirm you declined the call.

Switch to an incoming or on-hold call and put the current call on hold: Press the center button once. Press again to switch back to the first call.

Switch to an incoming or on-hold call and end the current call: Press and hold the center button for about two seconds, then let go. Two low beeps confirm you ended the first call.

Use Voice Control (iPhone 3GS only):  Press and hold the center button. See Voice Control.

If you get a call while the headset is plugged in, you can hear the ringtone through both the iPhone speaker and the headset.

Source: Apple.com