Who gets to decide who is a prophet and who is not?
God chooses who becomes a prophet. After all prophecy is a spiritual gift from God himself.
Let us look at Moses and the 70 Elders:
The Seventy Elders
16 So the Lord said to Moses, ‘Gather for me seventy of the elders of Israel, whom you know to be the elders of the people and officers over them; ...
Adam and Eve (in Genesis)
Cain (in genesis)
Noah (in Genesis)
Abraham (in Genesis)
Satan (in Job)
Job, Eliphaz, Bildad, and Zophar, and possibly Elihu (in Job, from a whirlwind)
Moses (in Exodus, Numbers, possibly Deuteronomy)
Aaron and Miriam (in Numbers 12)
Samuel (in 1 Samuel)
Elijah (the still small voice of 1 Kings 19)
King Belshazzar and his banquet ...
The audience at Jesus' baptism.
While he was still speaking, a bright cloud overshadowed them, and
behold, a voice out of the cloud said, “This is My beloved Son, with
whom I am well-pleased; listen to Him!” Matt. 17:5
All the people that Jesus "talked to" in the Gospels were "talked to" by God, since Jesus is God. In addition, in Acts, Jesus appeared in a blinding apparition to Saul (who became Paul) and talked to him. That is why we call Paul the "post-resurrection apostle."