The trumpet is a symbol that signifies a very clear sounding message being spoken to you, a very unmistakable message that you hear in the Spirit, and it is a Voice that you will recognize and never forget. It's not a real trumpet that you blow, but a clear sounding message that is getting your attention to what is being spoken to you. Trumpets were used to sound the alarm and to call the people to assembly for a purpose, along with many other things, and in Revelation it is a symbol that you must be able to discern - along with the rest of the book.1:10 I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet, Are you in the spirit enabling you to hear God’s trumpet?
4:1 After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter.