The authors of the Major prophets (Isaiah, Jeremiah and Ezekiel) were described or referred to by a number of terms due to the nature of their ministry and calling. They were called prophets, seers, watchmen, men of God, messengers, and servants of the Lord.
First, they functioned as preachers who expounded and interpreted the Mosaic law to the nation.
Second, they functioned as predictors who announced coming judgment, deliverance, and events relating to the Messiah and His kingdom.
Finally, they functioned as watchmen over the people of Israel. Ezekiel stood as a watchman on the walls of Zion ready to trumpet a warning against religious apostasy.