Because this poem is so short, the author picks his words very, very carefully to enhance the uncanny atmosphere through details that count. The first detail that creates the uncanny atmosphere is the fact that the author chose to have the visitor arrive at the house at night. This is a bit strange--who is out at night, travelling? But, this traveller is, and that is a detail that the author deliberately chose. It is dark, the moon is out, and that sets up for a creepy and scary scene.
The next detail is that the traveller receives no answer to his knock. Having no one answer enhances the eerie mood; one wonders what is going on, and why it is so quiet. As he waits, a bird startles and flies over his head. Imagine being there alone, in the dark, and having a random bird fly over your head--that would scare anyone. That detail also enhances the tense mood. Then, the author describes "a host of phantom listeners" that "throng" the hallway. Instead of having just one ghost or phantom, he has a lot of them, so many that they are thronging and filling the house. That's pretty creepy. The author also emphasizes the emptiness of the house, its silence, and that there is no one really there, besides the listeners. That makes it even more strange.
Through all of these carefully picked details, the author succeeds in creating a very uncanny and eerie atmosphere in "The Listeners." I hope that helped; good luck!