The first recorded time that God spoke to Moses is described in Exodus 3:1-6.
Now Moses was keeping the flock of his father-in-law, Jethro, the priest of Midian, and he led his flock to the west side of the wilderness and came to Horeb, the mountain of God. 2 And the angel of the Lord appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed. 3 And Moses said, “I will turn aside to see this great sight, why the bush is not burned.” 4 When the Lord saw that he turned aside to see, God called to him out of the bush, “Moses, Moses!” And he said, “Here I am.” 5 Then he said, “Do not come near; take your sandals off your feet, for the place on which you are standing is holy ground.” 6 And he said, “I am the God of your father, the God of Abraham, the God of Isaac
So he was in Midian and God spoke to him via an angel. See also Acts 7:30-34.
This occurred when Moses was eighty years old (Acts 7:23,30). It is possible that God had called him earlier, when he lived in Egypt, but it is not recorded.