recent report by Gartner shows the VoIP market is

recent report by Gartner shows the VoIP market is growing at a 36% compound annual growth rate inNorth America through 2015 to $2.2 billion•Moreover, the total U.S. telecom industry revenue will reach to $337 billion by 2015. That means the VoIPtechnology will represent less than one percent of U.S. telecom industry•That makes VoIP services an important revenue stream for many providers, but insignificant from anindustry-wide perspective. To a large extent, VoIP services is a product of high relevance for firms that sellto small and mid-sized businesses, with some partial-deployment use cases for enterprises•According to Infonetics Research, the residential segment continues to make up the majority of VoIPservices revenue, but the real growth is in the business segment, particularly SIP trunking services andhosted VoIP and unified communication services•Revenue from SIP trunking services to businesses is forecast by Infonetics to grow at a phenomenal 52%compound annual growth rate (CAGR) from 2011 to 2015•The following chart shows the revenue pie in VoIP services:

It's dated as well . But interesting .
http://www.academia.edu/4662449/Voice_over_IP_VoIP_