It is the accomplishment of certain events that indicates the transition from one of the seven angels to another.
The seventh is like each of the preceding six in that it is the trumpet of an angel, and in that it is a symbolic and not a literal trumpet. Revelation 8, 9, 10, 11.
But the trumpet which awakens the dead is not blown by an angel, but by the Son of God himself.
It is not a symbolic trumpet, for it is literally heard by the inhabitants of the earth. And he shall send his angels with a great sound of a trumpet, and they 
shall gather together his elect from the four winds, from one end of heaven to the other. Matthew 24:31;
And the LORD shall be seen over them, and his arrow shall go forth as the lightning: and the Lord GOD shall blow the trumpet, and shall go with whirlwinds of the south. Zechariah 9:14; For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: 1 Thessalonians 4:16.
It is called the last trump because when the Almighty descended upon Mount Sinai, in glory and majesty, like our Lord’s Second Advent the trump of God was heard, as it will be once more when the dead are raised. Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed, 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. 1 Corinthians 15:51,52."
J.N. Andrews