As with any of these kind of questions, it is very important to read the word or phrase you are trying to define in the context of the poem as a whole and especially the sentence in which it occurs. Contextual clues can help reveal the meaning of such phrases, and so if you can detect those contextual clues this will help you greatly in your understanding of such works of literature. Let us think about the immediate context of this phrase then:No mockeries now for them; no prayers nor bells; Nor any voice of mourning save the choirs, –The shrill, dementedchoirs of wailing shells; And bugles calling for them from sad shires.
Thus the "bugles" come as part of the poem that mourns angrily how the dead soldiers are only remembered by the "choirs" of "wailing shells" and the "bugles calling for them from sad shires." The bugle is a musical instrument that is normally played at military funerals, and thus the author is imagining the funerals that these dead soldiers will have back in England. As "taps" means a bugle call sounded at military funerals, this is the best answer to your question.