This is a very convenient feature for people who are not at their desks mostly.
You give a list of phone numbers that will ring simultaneously if your extension is either unanswered or busy. Set the time period for your extension to ring. After this time, the list of numbers will begin to ring. Set the time for how long they are going to ring. The list of numbers can be other extensions or they can be your cell phone or home phone or any other phone of your choice.
You can control these settings yourself from the Voicemail and Recordings menu.