How to get notification in Active Object Design Pattern

Detail:

Suppose I have calc(int, int) method in Proxy and also in Servant. When I call Proxy.calc(a, b) in thread1, this calculate job will be enqueued to the Scheduler, and then the Scheduler will do dispatch in thread2, and call Servant's calc method. My question is: How does the Proxy know the result of calc in Servant?
Or I don't understand Active Object Design Pattern rightly?