What is God's trumpet reserved for?

God's trumpet is reserved exclusively for the calling out of the Body of Christ, the event connected with Paul's teaching on the rapture in 1 Corinthians 15. It is singular, belonging to God Himself, not one of the seven angelic trumpets in Revelation 8, which occur at about the midpoint.

Les explains that many people want to connect the trump of God in Paul’s teaching about the rapture with the trumpets in Revelation, which occur around the midpoint. But in Revelation chapter 8, verse 2, seven angels were given seven trumpets, plural—not the trump of God, singular. Those are angelic trumpets, blown by angels, seven and not one. Les emphasizes that the seven angelic trumpets cannot be part of the singular trump of God Himself. God has a trumpet reserved, made however He makes trumpets, for just this event and this alone: the calling out of the Body of Christ. Les also notes that the Roman army used the trumpet to put out commands, much like the American cavalry did.

1 Corinthians 15:51

Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,

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Why can't the seven angelic trumpets in Revelation be the trump of God in 1 Corinthians 15?

They cannot be the same because Revelation 8:2 gives seven angels seven trumpets, plural, while 1 Corinthians 15 speaks of the trump of God, singular. These are angelic trumpets blown by angels, seven rather than one. God's trumpet is reserved for His own event, the calling out of the Body of Christ.