In Numbers 10, Yahweh instructs Moses to make two silver trumpets. They have several uses.
1) If both are blow, the people assemble at the door of the tent (v. 3).
2) A single blast is a summons only to the leaders (v. 4).
3) A second blast is an announcement that the people should set out, starting with the southern tribes (v. 6).
4) Trumpet blasts functioned as a memorial as Israel went to war, calling on Yahweh to remember His people and fight for them (v. 9).
5) Festivals were announced with trumpets (v. 10). At least on feast days, perhaps every day, ascensions and sacrifices were accompanied by trumpet blasts (v. 10).
These different uses illuminate the trumpet voices in the early chapters of Revelation. John hears a voice like a trumpet while on Patmos, a single blast that summons the leaders of the church. He hears a second blast after the seven letters, and this second blast summons him through the door of the heavenly sanctuary. In Numbers 10, the second blast calls Israel to break camp, to set out on a journey, and that is what the second trumpet means to John as well: He ascends into the sky and begins his journey that takes him through the wilderness to a high mountain.