To record an active call, tap "More" and select "Start Recording." A blinking red dot will appear next to the call timer. To end the recording, simply "End Call" or select "Stop Recording."
Recorded calls can be listened to by going to the Call History page. Find the call, marked by a red dot, and then press the blue > arrow to go to the call details. Press "Listen to Call Recording" to listen to the call.
You can also manage the recording using iTunes® or the Finder on your computer.