I don't think this is the DTMF tones for the number being dialed. I have perfect pitch (when I hear musical pitches, I can identify the note being played), and the tonal sequence seems to be identical between any of my speed dials.
The interesting thing I noted was the volume is actually changed on the phone. The volume slider moves to the mid-point to play the tone, then returns to whatever volume you were using.
Too funny how this forum brings things to my attention that I normally would have never thought about (though this is only my second day with my new 8310!).