How is the theme of conflict shown in the poem "The Voice" by Thomas Hardy?

The conflict in the poem is the speaker's grief. He misses his wife. In the context of the poem, she is simply gone. But as this is most likely Hardy's way of expressing grief after his wife had died, the woman in the poem is deceased. Hardy was known for intellectualism and avoiding blunt sentimentality. He manages to showcase this but allows for some sentimental notions.

The poem also shows a secondary conflict within the...

The conflict in the poem is the speaker's grief. He misses his wife. In the context of the poem, she is simply gone. But as this is most likely Hardy's way of expressing grief after his wife had died, the woman in the poem is deceased. Hardy was known for intellectualism and avoiding blunt sentimentality. He manages to showcase this but allows for some sentimental notions.

The poem also shows a secondary conflict within the grief itself. The speaker (for all intents and purposes, Hardy) clearly was more in love with his wife at an earlier time. He and she had become estranged, their love fading over time. This estrangement is why the speaker uses "woman" instead of something like "darling" or "beloved." Despite the estrangement, the speaker still does miss her. He misses her in the earlier part of their relationship when things were better and more loving. This is the secondary conflict. He is grieving but grieving more for a former, older version of her. 

In the first stanza, he imagines her calling to him, saying she is not like she was when they fell out of love (so to speak). He imagines her telling him that she is now (in death/spirit) like she was when they first were in love, "when our day was fair." 

In the second stanza, he wonders if he hears her, then asks to see her. In the third stanza, he adds that this willed hallucination might simply be the wind. He is left with only an imagined woman calling him. He mourns her death; this is the first conflict. The secondary conflict is the fact that he mourns her even though they had drifted apart. He grieves more for the earlier version of her. He wants this "version" (for lack of a better term) to be the one "calling" to him.