There are certain things that are always sexy--at least until the moment of disillusionment comes, but until that moment comes, our imaginations run wild. Perhaps the greatest of instrument of sexiness that a woman can control is our imagination. It's why some people apply class distinctions between lingerie models and strippers. One is classy, and the other is classless.
That's why something as disembodied as a voice can be extremely sexy. With no frame of reference, we are free to imagine just how utterly hot the speaker must be in order to produce such a sexy sound. This must be the reason why phone sex lines are still around even today. Something about a woman saying naughty things to you is very...thrilling.
But then things fall apart as soon as you see her face.
Then again, there are also voices that are just plain annoying and bitchy as hell. No matter how hot you are, if listening to you speak is akin to putting a cheese grater to my ears, consider my ass gone.