Bamidbar 10:9 presents the mitzva of sounding trumpets during wartime (“When you are at war in your land”), and during a time of trouble (“against an enemy who oppresses you”). Some require that both these conditions be present for the mitzva to be in effect. This leads the Avnei Nezer to ask whether we should blow the trumpets only for a voluntary war, or also for a milchemet mitzva (obligatory war). After all, since G-d has guaranteed us a successful outcome, one might posit that it is not considered a time of trouble. During the war against Jericho (which was a milchemet mitzva), they blew the shofar and not the trumpets (Yehoshua 6:2). This would seem to prove that blowing the trumpets is limited to a voluntary war.
While some limit the trumpet-blowing to a voluntary war, others offer a different limitation. The Pri Megadim points out that the verse uses the word “be-artzechem” (“in your Land”). He explains that this is the reason that in his time (18th century) the trumpet was not blown for trouble, as this was limited to trouble in the Land of Israel (or, by extension, trouble for the majority of the world’s Jews).
With this background, we can understand why Rav Shraga Feivel Frank (HaMa’ayan, 1970) exhorted people to blow trumpets near the Kotel in contemporary times of trouble. He argued that this would fulfill the mitzva.
In wartime, the trumpets are sounded as part of a special prayer service designed for this purpose. This prayer service is similar to that of Mussaf on Rosh Hashanah, with verses of Malchuyot (G-d’s kingship), Zichronot (asking G-d to remember), and Shofarot (about times when a shofar was sounded). Some maintain that the trumpets are blown in the battlefield itself, as we see from historical descriptions of the wars of the Maccabees.
Similarly, when our soldiers return from war or when they celebrate victory, they should celebrate and sound the trumpet. This is what King Yehoshaphat did when he returned victorious from the wars against Ammon and Moab. As it states, “For G-d had given them cause for rejoicing over their enemies. They came to Jerusalem to the house of G-d, to the accompaniment of harps, lyres, and trumpets” (II Divrei HaYamim 20:27-28). As a result, “The terror of G-d seized all kingdoms of the land when they heard that G-d had fought the enemies of Israel. The kingdom of Yehoshaphat was untroubled, and his G-d granted him respite on all sides” (ibid. 29-30).