These people http://www.spinvox.com/ do exactly that. The answer is that it works quite well, but maybe not well enough to rely upon. It struggles with people's names, jargon and noisy environments as you might imagine. Also there's been a few occasions where it missed a vital word and changed the whole sense of the message "Hi, I'm calling to let you know I'm going to be at the station tonight as planned" - missing a NOT. Ooops.