From Point-topic.com

Point Topic estimates that over 11 million people were using a retail voice over IP (VoIP) service for at least some of their telephone calls at the end of March 2005. That’s an increase from just over 5 million at mid-2004 according to Point Topic research published in December 2004 and the Point Topic VoIP Application Source profiles in Broadband Money Makers.

This number is based on a total of publicly available figures for VoIP subscribers, combined with estimates where subscriber numbers were not available. Many of these estimates are based on a conservative 10% growth from Q4 2004 to Q1 2005. The total does not include PC-based ‘soft-client’ services like Skype and VoiceGlo.