In first Samuel 3, God is calling Samuel. At first Samuel thinks that Eli is calling him.
After Samuel hears the voice calling him for a third time, Eli realizes that it was the
voice of God Samuel was hearing. He told Samuel,  "Go and lie down, and if he
 calls you, say, 'Speak, Lord, for your servant is listening.' So Samuel went and lay
down in his place." (First Samuel 3:9 NIV)  When God called Samuel again,
Samuel answered, "Speak, Lord, for your servant is listening." Samuel grew up to
be a great prophet of the Lord. He eventually led the Isrealites in battle defeating
the Philistines.