The speaker begins the poem by addressing the owner of the "voice" of the title directly, an example of apostrophe: "Woman," he says, calling out to her just as she seems to repeatedly "call to" him.
The speaker specifically calls this person "Woman much missed," a phrase filled with humming /m/ consonance and alliteration. The very sounds of the poem call attention to themselves, subtly conveying the power of this mysterious voice.
The speaker then says to this voice: "how you call to me, call to me." Notice that he doesn't just say "you call to me" but "how you call to me." Apparently, there's something about the way that this voice calls to the speaker that, as the reader will see, moves him profoundly.
The repetition of "call to me" (an example of the device epizeuxis) creates an echo, which has a ghostly, unsettling effect (perhaps calling to mind the way a sound bounces in a dark, empty cave). It also suggests that the voice calls again and again, as though desperate to say something to the speaker.
The next line is also quite repetitive. The contrast between those two "you"s—"now you are" and "as you were"—emphasizes the fact that the woman this voice belongs to changed quite a bit over the course of her relationship with the speaker.
Finally, these lines also establish the poem's general meter: dactylic tetrameter. A dactyl is a poetic foot with one stressed followed by two unstressed syllables: DUM-da-da. It thus sounds a bit like an echo too, a strong syllable fading away into two weaker ones:
Woman much | missed, how you | call to me, | call to me,
The second and fourth lines of the first three stanzas are shortened, missing their two weak syllables at the end:
Saying that | now you are | not as you | were
Still, all these dactyls have a kind of lingering, stretched quality. The meter seems to capture the way that the speaker's feelings of longing and regret have no real place to go, simply petering out on the page.