“God called to him out of the midst of the bush.” We learned in Exodus 3:2 that it was an angel that appeared to Moses. Yet this verse says “God called.” God called via His agent, the angel. It often occurs in Scripture that God acts through his agents and the Bible says that God was the one who acted. However, it is also possible that at some point in the record God did speak audibly to Moses.
“Here I am.” This is an idiom. Moses is not telling God that he is there; this is the equivalent of answering God’s call with “Yes,” or “It is me.”