--- Source: Stefana Broadbent, an anthropologist who leads the User Adoption Lab at Swisscom, cited by the Economist in Tech Quaterly story on June 9, 2007: Home truths about telecoms.
They also quote her thus: "The most fascinating discovery I've made this year is a flattening in voice communication and an increase in written channels. . . . Users are showing a growing preference for semi-synchronous writing over synchronous voice." The Economist's gloss: "Her research in Switzerland and France found that even when people are given unlimited cheap or free calls, the number and length of calls does not increase significantly. This may be because there is only so much time you can spend talking; and when you are on the phone it is harder to do other things. Written channels such as e-mail, text-messaging and IM, by contrast, are discreet and allow contact to be continuous during the day."
It seems writing really is a useful alternative channel. I guess there's a reason why the Blackberry was so succesful.