http://www.mangago.me/read-manga/ai_wo_choudai/an/ai-wo-choudai-chapter-5.html/33/ i dont...

His bother, the teacher, was on the phone while they were having sex. The sound effects were the sound of him hanging up. In Europe and Asia, their dial tones are completely different. There are 3 beeps when someone hangs up.

I think it's a metaphor. It's not a real bomb on his chest, but it's his desire for him that will make him explode...Or they died.