This feature notifies you (with a beep) when there's a second incoming call, while you're on another call. Switch to the new phone call without disconnecting the first call.  

How it works:

• When you hear the notification (beep), simply press the "Hold" button (select Line 2) or the "flash" button or quickly press the switch hook
  
• You can take the second call while the first caller is on hold
• When finished with the second call, press the Line 1 button or flash button or the switch hook again, to go back to the first caller.

You don't have to worry about tying up your phone line, even if you are waiting for an important call.