For those who don't understand this is my interpretation of the story. There are two different timelines. the present timeline is the one where Yeon-Ho is an adult and it's been 10 years since his friend died. The other timeline is where Jae-ha is still alive. The telephone in the room that Yeon-Ho finds is the only way to contact Jae-ha. When Yeon-ho confesses to Jae-ha about how he died and that he cannot go to the graduation and to meet him, Jae-ha does so. And in doing so Jae-ha was saved and that past timeline then became the present timeline. When Yeon-Ho goes out for a date and sees Jae-ha he runs towards him not realizing the truck and gets hit but survives, I say this because the author did not show a clear definition of him dying and also in the extra the author showed examples of Yeon-ho surviving never once dying. When we see Yeon-ho wake up as his younger self crying about a dream he had, that is the second timeline. The younger Yeon-ho memories were connected to his older self in the future which made it seem like a dream. I know that neither will die now because of the scene where Yeon-ho saved Jae-ha from getting hit by the car in their younger selves. They will always be together now.
For those who don't understand this is my interpretation of the story. There are two different timelines. the present timeline is the one where Yeon-Ho is an adult and it's been 10 years since his friend died. The other timeline is where Jae-ha is still alive. The telephone in the room that Yeon-Ho finds is the only way to contact Jae-ha. When Yeon-ho confesses to Jae-ha about how he died and that he cannot go to the graduation and to meet him, Jae-ha does so. And in doing so Jae-ha was saved and that past timeline then became the present timeline. When Yeon-Ho goes out for a date and sees Jae-ha he runs towards him not realizing the truck and gets hit but survives, I say this because the author did not show a clear definition of him dying and also in the extra the author showed examples of Yeon-ho surviving never once dying. When we see Yeon-ho wake up as his younger self crying about a dream he had, that is the second timeline. The younger Yeon-ho memories were connected to his older self in the future which made it seem like a dream. I know that neither will die now because of the scene where Yeon-ho saved Jae-ha from getting hit by the car in their younger selves. They will always be together now.