Answer
I think etymologically the word, or the most common word, for prophet in the Old Testament, nabi, is correctly understood as a passive noun form from a verb which must have meant "to call," and so a prophet was one who was called. In context of the prophets, prophets are those who are sent; that is, they are called in the sense that they are called to a commission. Prophets are commissioned. But in actual usage, I think when the Old Testament uses the term nabi, it's especially in close association with the prophet speaking and speaking the words of the Lord.
Answer by Dr. Douglas Gropp
Dr. Douglas Gropp was formerly Professor of Old Testament and Associate Academic Dean at Redeemer Seminary