In the visual novel, this is the ringtone used for Moeka Kiryu's phone. Availability
Featured on the Steins;Gate (Original Soundtrack) .
STEINS;GATE Original Soundtrack - Compilation by Various Artists
Included on Disc 2 of the Steins;Gate Original Soundtrack + Radio CD and recent vinyl releases.
There is also an orchestrated version titled from the Steins;Gate Symphonic Reunion album, which incorporates this theme along with other "Ringing" tracks.