What's this anime film involving a phone's ringtone?
Harmonie is the anime you're looking for.

Synopsis (MyAnimeList)

Every person has his or her own little world. Among the 34 bustling students of classroom 2-1, there are many such worlds.
Honjou Akio's world—my world—is filled with passionate talk about last night's anime with Yoshida, Watanabe, and other friends. It's an fun, comfortable place... But, I always wonder. Wonder about the world of Makina Juri, smiling and surrounded by gorgeous friends on the other side of the classroom.
One day, I enter her world. But it's not as simple as it sounds.

Synopsis (IMDB)

A girl's phone rings during a class. A boy tries to reproduce the ringtone and this becomes the spark that makes them meet, dream together and discover each other personal world.