Description of call model:

Incoming Call:

	Channel backend waits for a RING or equivalent on some sort of
interface. Typically this is done in its own thread.  When a RING is
detected, the backend should create a channel structure and then call
ast_pbx_start() on that channel, which will create a thread to monitor
that interface.  At this point, the PBX and/or applications it launches
will manage the interface, and it need not be monitored by the
aforementioned thread.  When the applications are finished, the requisite
hangup function will be called, at which the channel can be considered to
be no longer valid, and the thread that controls it will imminently be
terminated. 

