I find most popular songs are too quiet for me to use as a ringtone - I never hear them before the call has rolled over to voicemail. If I could get a decent rendering of "O Fortuna" (think of the movie Excalibur) I might go with that.
One thing that's given me a start a couple of times is Nokia's default text message beep. It's Morse code for SMS (... -- ...) which is almost identical to SOS (... --- ...). I'll be at the store or at work and hear what sounds like somebody sending SOS, and immediately think I should call the Coast Guard.
Dave
