The Bible is full of people hearing and answering calls; Noah to build the Ark, Moses to lead his people, the prophets, and the Apostles. All of these people heard the Lord's call and said; "Here I am".
Our own history is full of people who heard a call to serve God and humanity; the Abolitionists, Martin Luther King Jr, Desmond Tutu, and Father Roy. We too are called to do God's will.
Of course, knowing what that will is can take some time. It can also be ignored or placed on the "back burner". Such is the case with me; but I think somehow that call will have me working in the inner-city of Harrisburg PA. I just need to figure what exactly.