Uhm. From how i understood it, its about the phone call but not truly. The core of this is that Jay believes Yoohan to be conflicted about the direction of his life, his regrets towards his family, and whether he should continue living or just die. At that moment, Yoohan was at the verge of death and he considered death, maybe he was unable to pick up, maybe he actually could. The point is, during that time hes still thinking of "atoning" for his sins. So Jay got triggered and thought, at the end of his wit, well why not share some of that guilt towards me *cuts himself* this is a desperate action Jay did, as Yoohan's lover I'd say its a moment or one of the many which put an end for Yoohan's inner conflict, he chose Jay and himself in the end and looked forward again.

but you know, i keep reading the part when jay cuts himself during the whole yoohan stabbing incident. it still leaves me a big question mark
so he rlly did cut himself himself Infront of yoohan because
1. he thought yoohan was not picking up his call because he couldn't do so (cs he's been stabbed) or
2. if yoohan intentionally didn't pick up so jay thought he wanted to be with his dead family instead ?
like he rlly grabbed a knife because he didn't like the 2nd option at all? cs if it was the 2nd i mean i don't mind, jay was always described as a lunatic but it just got me thinking Abt that particular scene the most