First What does a trumpet represent in the scripture? Paul says in one scripture:
1 Corinthians 14:8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
Israel had a trumpets and each sound had a certain meaning:
2 Make thee two trumpets of silver; of a whole piece shalt thou make them: that thou mayest use them for the calling of the assembly, and for the journeying of the camps.
3 And when they shall blow with them, all the assembly shall assemble themselves to thee at the door of the tabernacle of the congregation.
4 And if they blow but with one trumpet, then the princes, which are heads of the thousands of Israel, shall gather themselves unto thee.
5 When ye blow an alarm, then the camps that lie on the east parts shall go forward.
6 When ye blow an alarm the second time, then the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys.
7 But when the congregation is to be gathered together, ye shall blow, but ye shall not sound an alarm.
8 And the sons of Aaron, the priests, shall blow with the trumpets; and they shall be to you for an ordinance for ever throughout your generations.
9 And if ye go to war in your land against the enemy that oppresseth you, then ye shall blow an alarm with the trumpets; and ye shall be remembered before the LORD your God, and ye shall be saved from your enemies.
10 Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, ye shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your God: I am the LORD your God.
The trump calls in the old testament are types and shadows of the new:
In their marching to capture Jericho the children of Israel the had seven priests who carried the seven trumpets for battle.
3 And ye shall compass the city, all ye men of war, and go round about the city once. Thus shalt thou do six days. 4 And seven priests shall bear before the ark seven trumpets of rams' horns: and the seventh day ye shall compass the city seven times, and the priests shall blow with the trumpets. 5 And it shall come to pass, that when they make a long blast with the ram's horn, and when ye hear the sound of the trumpet, all the people shall shout with a great shout; and the wall of the city shall fall down flat, and the people shall ascend up every man straight before him. 6 And Joshua the son of Nun called the priests, and said unto them, Take up the ark of the covenant, and let seven priests bear seven trumpets of rams' horns before the ark of the LORD. 8 And it came to pass, when Joshua had spoken unto the people, that the seven priests bearing the seven trumpets of rams' horns passed on before the LORD, and blew with the trumpets: and the ark of the covenant of the LORD followed them. 9 And the armed men went before the priests that blew with the trumpets, and the rereward came after the ark, the priests going on, and blowing with the trumpets. 10 And Joshua had commanded the people, saying, Ye shall not shout, nor make any noise with your voice, neither shall any word proceed out of your mouth, until the day I bid you shout; then shall ye shout. 11 So the ark of the LORD compassed the city, going about it once: and they came into the camp, and lodged in the camp. 12 And Joshua rose early in the morning, and the priests took up the ark of the LORD. 13 And seven priests bearing seven trumpets of rams' horns before the ark of the LORD went on continually, and blew with the trumpets: and the armed men went before them; but the rereward came after the ark of the LORD, the priests going on, and blowing with the trumpets. 14 And the second day they compassed the city once, and returned into the camp: so they did six days. 15 And it came to pass on the seventh day, that they rose early about the dawning of the day, and compassed the city after the same manner seven times: only on that day they compassed the city seven times. 16 And it came to pass at the seventh time, when the priests blew with the trumpets, Joshua said unto the people, Shout; for the LORD hath given you the city.
20 So the people shouted when the priests blew with the trumpets: and it came to pass, when the people heard the sound of the trumpet, and the people shouted with a great shout, that the wall fell down flat, so that the people went up into the city, every man straight before him, and they took the city.
According to the scripture, Israel will be gathered by the Great trumpet to worship God at the temple mount:
12 And it shall come to pass in that day, that the LORD shall beat off from the channel of the river unto the stream of Egypt, and ye shall be gathered one by one, O ye children of Israel.
13 And it shall come to pass in that day, that the great trumpet shall be blown, and they shall come which were ready to perish in the land of Assyria, and the outcasts in the land of Egypt, and shall worship the LORD in the holy mount at Jerusalem.
There is the trump that will result in the resurrection of the dead and the catching away of the bride:
1 Corinthians 15:52 In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
John when he was about to receive a message to the seven churches heard a voice of the trump speaking to him from the earth.
10 I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet,
11 Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea.
At the end of the message to the seven churches he heard the same voice of a trump calling from heaven calling him to come up hither he was brought before the throne:
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.
2 And immediately I was in the spirit; and, behold, a throne was set in heaven, and one sat on the throne.
The last trump is the trump that will be sounded after the ending of the gospel dispensation, at the dispensation of the fullness of time, the trump that will gather us unto him. It is a call to gather his saints to him at the time of his coming:
10 That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:
3 Our God shall come, and shall not keep silence: a fire shall devour before him, and it shall be very tempestuous round about him.
4 He shall call to the heavens from above, and to the earth, that he may judge his people.
5 Gather my saints together unto me; those that have made a covenant with me by sacrifice.
It is the call to gather his saints, both the dead and the living, just before he brings judgment to the world of the ungodly
19 Thy dead men shall live, together with my dead body shall they arise. Awake and sing, ye that dwell in dust: for thy dew is as the dew of herbs, and the earth shall cast out the dead.
20 Come, my people, enter thou into thy chambers, and shut thy doors about thee: hide thyself as it were for a little moment, until the indignation be overpast.
21 For, behold, the LORD cometh out of his place to punish the inhabitants of the earth for their iniquity: the earth also shall disclose her blood, and shall no more cover her slain.
It is the trump that will go forth on the day of redemption, just before the wrath of the Lamb.
19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God.
21 Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.
23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body
1 Who is this that cometh from Edom, with dyed garments from Bozrah? this that is glorious in his apparel, travelling in the greatness of his strength? I that speak in righteousness, mighty to save.
2 Wherefore art thou red in thine apparel, and thy garments like him that treadeth in the winefat?
3 I have trodden the winepress alone; and of the people there was none with me: for I will tread them in mine anger, and trample them in my fury; and their blood shall be sprinkled upon my garments, and I will stain all my raiment.
4 For the day of vengeance is in mine heart, and the year of my redeemed is come.