Once the caller has requested CC, then Asterisk's job is to monitor the progress of the called parties. It is at this point that Asterisk allocates the necessary resources to monitor the called parties.
A generic monitor uses Asterisk's device state subsystem in order to determine when the called party has become available. For both CCBS and CCNR, Asterisk simply waits for the phone's state to change to a "not in use" state from a different state. Once this happens, then Asterisk will consider the called party to be available and will alert the caller.
A native monitor relies on the network to send a protocol-specific message when the called party has become available. When Asterisk receives such a message, it will consider the called party to be available and will alert the caller.
Note that since a single caller may dial multiple parties, a monitor is used for each called party. It is within reason that different called parties will use different types of monitors for the same CC request.
lmadsen 2010-10-21