"Trumpet" - Heb. "Hatzotzra" from the verb "hatzar," meaning to warn, to proclaim; a warning, a proclamation.

"The trumpet of God" is the proclamation, the announcement to the nations of God's Will, His Commandments by the Patriarchs, the Prophets, Jesus Christ, the Apostles, all the Righteous.

It is the "Voice" of Truth, sounding from the mouths of all the Righteous throughout history.