Hi Sanguine ,
The method call_outbound_plug is responsible for calling a particular implementation method defined for an outbound plug (OP_prefix for O/P , gets created when you create an outbound plug ) .
Whereas the method fire_outbound_plug is responsible for actually triggering the navigation i.e whatever the o/p is destined to call will be called say for instance delegated o/p of window of another component .
Hope this will be helpful .
Thanks ,
Vaibhav