You might be surprised to find that the Tri-tone wasn't originally engineered for the iPhone. As Kelly Jacklin, the creator of the Tri-tone explains, the sound was originally intended as a confirmation sound for a disc burning feature in what would be the predecessor to iTunes.
I was looking for something "simple" that would grab the user's attention. I thought a simple sequence of notes, played with a clean-sounding instrument, would cut through the clutter of noise in a home or office. So I had two tasks: pick an instrument, and pick a sequence of notes. Simple, right? Yeah, says you; everyone's an armchair musician...
For reference, the Tri-tone is the default alert tone used when when receiving an SMS or Voicemail on an iPhone.