That conventional voice traffic would decline on international routes was known (except to the people who came with elaborate schemes to control it). How fast was where the guessing was. Telegeography has some numbers:
New data from TeleGeography show that growth in international call traffic has slumped while international traffic routed via Skype continues to accelerate. International phone traffic grew an estimated 4 percent in 2010, to 413 billion minutes, down from 5 percent growth in 2009, and a far cry from the 15 percent average growth rate achieved during the previous two decades.
Where did the growth go? One factor contributing to the slowdown is the deep recession of 2007-2009, which affected business demand for international communications and many consumers’ ability to pay for international telephone calls. Economic conditions are reflected in sharply reduced calling card volumes, and in a drop in traffic from the U.S. to Central America.