Koekoi(2015)

Koe Koi

Matsubara is the class rep of Yoshioka's new class. She got sick right after entering high school and has to rest at home. One day her class rep calls her to know about her state and she falls in love with his voice. However, when she meets Matsubara-kun, there's a something weird about him, she can't see his face!

(Source: Anime News Network)