"Dialtones begins with a brief preparation phase prior to its performance, during which the members of the audience register their wireless telephone numbers at a cluster of secure Web kiosks. In exchange for this information, the participants receive seating assignment tickets for the concert venue, and new "ringtones" are then automatically downloaded to their handsets. During the concert itself, the audience's mobile phones are brought to life by a small group of musicians, who perform the phones en masse by dialing them up with a specially designed, visual-musical software instrument. Because the audience's positions and sounds are known to the Dialtones computer system, the performers can create spatially-distributed melodies and chords, as well as novel textural phenomena like waves of polyphony which cascade across the crowd; these musical structures, moreover, are visualized by a large projection system connected to the performers' interfaces. Towards the end of its half-hour composition, Dialtones builds to a remarkable crescendo in which nearly two hundred mobile phones peal simultaneously..."
Check it out. They even have MP3s of the concert.