Between call-waiting and speech-based signals ("The number you are trying to reach is busy..."), the soothing sound of a busy phone is something you rarely hear these days. How many teenagers have heard this tone?  Hard to believe, but think about it - when is the last time you heard this tone when you made call. I cannot recall on my end - I would say it's been years.      Yep, I bet 10 years from now, this one will be gone too.