The voice speaking to Luke in the poem is being heard by Luke's tormented and despairing spirit, not by his ears. It has come to him "out of a grave" for the purpose of helping him to find the "one way to where she is."
"She," of course, is Luke's dead lover, the person whose loss is driving Luke to consideration of suicide as a means of escape from his grief. The speaker in the poem does not find anything wrong or sinful in this idea; indeed, the speaker encourages Luke to "go to the western gate" and "wait for what will come."
Perhaps the speaker is Satan, attempting to recruit another soul since "hell is more than half of paradise." In any case, the speaker's aim is to reassure Luke that his love is ready and waiting for him; all he has to do is follow the voice's directions. According to the voice, if Luke will "go" and "trust" that "she will call," their reunion is assured.