The headphones that come with an iPhone have a built-in microphone for hands-free calling. The middle is also a remote button – if you squeeze it you’ll hear a click.
If the headphones are plugged in when the phone is ringing, click the button once to answer the call. To reject the call, press and hold the button for two seconds, then release it. At the end of a phone conversation, click once to end the call.
More call controls
If you’re already on a call and another one comes in and you’d like to switch click the remote button once to go to the new call. Click again to go back to the original phone conversation.
To end the current call before switching, press and hold the button for two seconds then let go.
With the iPhone 4 and later you can use the Volume buttons to change the volume of the call.