He was only kid, just 12 or 13 years old. He was awakened repeatedly during the night, but had no clue who was calling until his caregiver Eli finally figured it out.

So Eli directed Samuel, “Go back and lie down. If the voice calls again, say, ‘Speak, God. I’m your servant, ready to listen.’” Samuel returned to his bed.

Then God came and stood before him exactly as before, calling out, “Samuel! Samuel!”

Samuel answered, “Speak. I’m your servant, ready to listen.” (Eugene Peterson's "The Message")