Here is what Telegeography has to say on the subject:
Computer-based Voice over IP (VoIP) is nothing new, but Skype is the first such service to break into the mainstream, attracting millions of users worldwide. Skype had 1 million simultaneous users within six months of the release of its first version for Windows in July 2004. By the end of the third quarter of 2006, Skype had 136 million registered users, and the number of users online now regularly exceeds 8 million. These users generated about 6.6 billion minutes of traffic in the third quarter of 2006, and are on track to make over 27 billion minutes of PC-to-PC calls this year. About half of Skype’s traffic is international.
This has prompted worries that Skype–and similar services–could undermine the viability of the international long-distance market. However, while the volume of international traffic routed via Skype is significant, the quantity is still small when compared to a global switched and VoIP traffic base of 264 billion minutes. Computer-to-computer traffic between Skype users in 2005 was equivalent to 2.9 percent of international carrier traffic in 2005 and approximately 4.4 percent of total international traffic in 2006. Furthermore, not all of Skype’s traffic is a net loss for international carriers. Skype also offers a paid “Skype Out” service, which allows Skype users to place calls to traditional telephones. The service relies on wholesale international carriers, including iBasis, Cable & Wireless, and Level 3, to terminate this traffic to the telephone network.
Still, it’s clear that VoIP services will continue to gain in popularity. “Someday, all calls will be routed over the Internet,” commented Stephan Beckert, Research Director at TeleGeography. “But the numbers suggest that traditional international carriers aren’t going to disappear anytime soon.”