When in your own land you go to war against an enemy that is attacking you, you shall sound the alarm on the trumpets, and the Lord, your God, will remember you and save you from your foes (Numbers 10:9).
In Jewish tradition, the trumpet or the shofar was blown to announce the new moon, month and year; to announce the coming of the Lord (recall how the Feast of Trumpets is celebrated before the Day of Atonement); to assemble the people for the Lord (the Jews even believed this would be the mechanism that summoned the dead to come to the final judgement); and to sound the alarm and begin the attack (recall the stories about the walls of Jericho and the other Old Testament battles where the Ark of the Covenant was carried out into battle).
The Book of Revelation gives us a clue regarding the trumpet. In two particular verses, John identifies the trumpet as the words of an angel: “I was caught up in spirit on the Lord’s day and heard behind me a voice like a trumpet saying, ‘Write what you see in a book and send it to the seven churches….’” (Rev 1:10). And again: “After this I looked, and behold in heaven an open door! And the first voice, which I had heard speaking to me like a trumpet said, ‘Come up here, and I will show you what must take place after this’” (Rev 4:1).