Hardy wasn't one of those poets who picked a form and stayed with it. Shakespeare loved those sonnets. Milton was pretty committed to blank verse. But Hardy was all over the place with his forms. He liked to change things up. "The Voice" is written mostly in dactylic tetrameter (not exactly a common form), while his famous poem "The Darkling Thrush" was written in good ol' iambic tetrameter (much more common).
Hardy felt that the form of the poem should fit its content, so, not surprisingly, he was always changing up his forms. In "The Voice," for example, the poem's dizzying form echoes its dizzying content. If you want another example, check out "The Convergence of the Twain," a poem about the Titanic for one strange-looking, yet poem-appropriate form. (Do the stanzas remind you of icebergs? Good.)
Hardy wasn't afraid to change the forms of his poems even while he was writing them, which is why the last stanza of "The Voice" looks so different from the rest. We like to think of Hardy's style as a sort of homegrown, do-it-yourself aesthetic: make up your own forms! Serve them with homemade fresh fruit compote! (If you're into that sort of thing). Don't be afraid to experiment!