Oh, this is just a little brilliant. Steve Reich is a composer famous for his experimentation with musical looping and phasing. His 1967 piece Piano Phase featured a pair of pianists repetitively performing the same piece at two slightly different tempos, forming a continually evolving musical round. Seth Kranzler took this idea and made a Reich-like piece with two iPhones ringing at slightly different tempos. Here's a video of the effect in action:
Man, this is nerdy on so many levels and I am here for it.
Update: Here's a couple more tracks along the same lines: the drum fill from In the Air Tonight and the Amen break.
(via @frank_chimero)