The verb for “were called” always implies a “summoner,” a leader (Judges 6:34, Judges 6:35; Judges 7:23, Judges 7:24; Judges 12:1; Judges 18:22, Judges 18:23. The Ammonites were thus gathered by a leader. The Israelites, however, “came together,” or “were gathered.” This hints at the problem that the Israelites had: they lacked a leader, as explicitly noted in Judges 10:18.
17 Then the Ammonites were called to arms, and they encamped in Gilead. And the people of Israel came together, and they encamped at Mizpah.