how to use CAMP-ON on freepbx for callback

HI, I am using freepbx [login to view URL] and not able to use camp-on module, so if have proper informaition please bid. here you need to explain 1. what is the main use of camp-on. 2. can we use this for auto call back? 3. how to configure this if its for auto call back. 4. can we make setting like if any extension is busy it will play a msg extension is busy please press 8 for auto call back. and when that extension will be an auto call back will be orginate to caller.