I've Been to Your Future (2023)

Both are about the ML in the future trying to prevent the FL's death in the past by connecting with her through her cellphone.

Both dramas are about one of the leads suddenly dying and the other attempting through time travel to save their life.

Both jump between past and present as one of the leads in the present tries to change the past of their high school years and prevent the death of the other lead.

Both are short length fantasy dramas with the same actress and actor playing the leads.
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Both dramas involve one of the leads dying and then a cellphone allowing the other to return to the past/alter the events that happened. Both have a school setting.