Kimi ni shika Kikoenai, Orange Recommendations
The 2007 version of the manga with the same name.
Both have similar themes and plots. In Kimi ni Shika Kikoenai, a girl, unable to get her own cellphone, imagines one in her mind. But suddenly, someone actually calls her--a woman; she also ends up talking with a boy whom she ends up having feelings for. Both manga have tragic elements and also have characters where the main character's future self contacts them.
Both are about teenagers who communicate over time to help a friend