Walk the halls of any decent sized software company. Stick your head into the offices of the code monkeys. I guarantee you a significant proportion of the phones will be set to divert to voicemail. If you were biassed and only visited the most productive people, there may even be a majority of them set that way.
The biggest enemy of creative thought is interruption. When you are in the flow, ideas and code come easily, and if that flow is disturbed it takes significant time to get back into the right frame of mind. The flow is almost addictive, and I for one enjoy being there. In that state, I'm much more productive. The same is most likely true of other creative types who spend their time at the screen - like writers.
That's why email has been so riotously successful - it's communication you can control. Geeks are big on control. It's also why instant messaging is dumb. It's interruptive and attention demanding. It's a telephone on steroids. And people want to add voice, even video ! By telephoning me or messaging me, you implicitly state that your time is more important than mine. You shouldn't be surprised to find that I disagree.
Microsoft thought IM was so important they incorporated into Windows XP, and even made it hard for you to de-install it. They think their audience consists primarily of the time-rich text-messaging generation: teenagers. It's evidence that Microsoft is turning its attention away from the office desk and the time-poor knowledge worker. Those folks are the ones that made Microsoft rich.