Wondering how to control the iphone 4 with only 3 buttons on the headphones?
Button Shortcuts for iPhone Headset
While there is an incoming call, you can:
1. Press once to pickup the call
2. Press twice to send the call to voicemail.
3. Press and hold center button for about two seconds to decline the call. You’ll also hear two low beeps that confirm you the call is declined.
While you’re in a call, you can simply press once to end the call.
While you’re in a call and there is a second incoming call, you can:
1. Press once to switch to another call and put the current call on hold. Press the button again to switch to the first call.
2. Press and hold the button for about two seconds to ignore the second call.
While you’re not in a call, you can press the button once to activate iPod and playback yourmusic.
While you’re listening to music in iPod, you can use the button to control the music playback:
1. Press once to pause the song and press again to resume it.
2. Press twice to skip to the next song
3. Press three time to go back to the previous song
For iPhone 3GS, the bundled headset is enhanced with volume control buttons. You can use the “+†and “-†buttons to adjust the volume. And, one more thing. You can press and hold the center button to activate voice control, which is a new feature comes with iPhone 3GS.
Source: http://www.simonblog.com/2009/07/29/get-to-know-the-iphone-headset-shortcuts/