During the opening of the third chapter, Naomi receives a call from her mother. She tries to answer her, but her voice doesn't go through, and eventually her mother's is drowned out by an unknown voice repeating the phrase, "Help me..." which causes Naomi to throw the phone down the stairs. However, upon realizing her mistake, Naomi chases after it to make sure it's not broken and ends up hurting herself in the process.
It can be found in the stairwell leading to the third floor bathrooms. After seeing a mirage of Naomi, Satoshi sees her cell phone flying down the stairs before landing near him. As he approaches, the phone begins to ring. Satoshi has the option to answer it or not. If he does answer, he will hear the same voice that Naomi had earlier, repeating the same phrase before hearing Naomi's mother. The voice asking for help continues, and Satoshi throws the phone to the ground. He then decides to take it with him before leaving.
The final time we see Naomi's cell phone is during the true end of the fifth chapter. After finding out the truth about Seiko's death, Naomi begins to succumb to the darkening, blaming herself for what happened. However, the unsent message on Seiko's phone begins to appear in her inbox. When Naomi blames herself more, she receives more messages from Seiko's spirit, reassuring her that what she did wasn't her fault. Due to the series of texts, Naomi is able to break free of the darkening and join her friends.