Now the boy Samuel was ministering to the Lord ... Samuel was lying down in the temple of the Lord, where the ark of God was. Then the Lord called Samuel, and he said, "Here I am!" and ran to Eli and said, "Here I am, for you called me."
And the Lord called Samuel again the third time. And he arose and went to Eli and said, "Here I am, for you called me." Then Eli perceived that the Lord was calling the boy. 1 Samuel 3:3-8
There is some obvious behind-the-scenes child training Samuel's parents had done in this young boy's life. Children left to themselves don't respond to authority in such a way. Behavior like this indicates that deliberate attention had been spent to get him to listen carefully, and respond immediately. God saw that training, obedience, and urgency, and used him because of it.
God doesn't call indolent folks—he calls those who respond immediately.