Fell upon Me

Both are mini dramas in which the leads are connected in different times by an object through which they can hear each other's voices, leading the one in the past to try to prevent the tragedy of the future.

"Time Travel" aspect in both are similar. And When FL returns to present, they both seem to realize differences in their original perceived memories....

Both of these dramas have similarities in the cinematography and the overall angsty vibe, and story wise they both are time related, where two different timeline can contact each other.

Very similar mystery vibes. Both have a time travel aspect, both have one character who is dead in the future, both are set in high school.

Both are time-travel and mystery dramas with a dash of romance.
Both include the death of a main character and another character trying to save their life.

Both are about the ML in the future communicating with the FL in her school days in the past through a cell phone.

Both dramas are about the leads in parallel/different timelines - ML in the future, FL in the past - communicating through a cellphone.