It is a prophetic voice that speaks for God, and His prophets will always have as the basis of their prophecy the commandments of God as evidence. The message they give (predictive or not) will always be in harmony with previously revealed truth, even though the prophet may be breaking new doctrinal ground, which happens now and then.
We can see another difference between a prophet and a priest or minister. The priest or minister conserves old truth and implements new truth given by the prophet. Most of the time new truth will come through a prophet. Under the New Covenant, of course, new truth came through apostles who were about as close to prophets as one can get without being prophets. Paul makes that clear when he lists the offices in the church, listing apostles first and prophets second (Ephesians 4:11). Once we leave the Old Covenant for the New, God uses apostles to announce new truth, and the prophet is moved into a secondary position. However, throughout the Old Testament, new truth or new doctrines came through prophets.
A minister's job is to conserve what has already been given, to hold fast to what was given in the past, and to recognize that new truth comes through an apostle. There is no apostle now, so we should not expect that there will be any new truth. However, if God raises up a prophet, then we also have to recognize that new truth can come through him. He will not break God's pattern. New truth will either come through an apostle or a prophet. The prophet breaks new ground, yet he also conserves the old.
There is a difference between a minister and a prophet. A minister does not give new truth but conserves old truth. The prophet or the apostle will conserve the old and also proclaim the new.