13Then the sixth angel blew his trumpet, and I heard a voice from the four horns of the golden altar before God,
Each of the seven angels (see Revelation 8:2, Revelation 8:6) blew his trumpet in response to the prayers of the saints (Revelation 8:3–4). See the commentary on Revelation 8:3–4 for why the prayers of the saints triggered these trumpet blasts.
13 Then the sixth angel blew his trumpet, and I heard a voice from the four horns of the golden altar before God,