One deficiency is that the article doesn’t really encompass the whole picture necessary for putting in VoIP. For example, most installers would consider using conventional PSTN phone lines or a T-1 connection for multiple lines, rather than attempt to use public IP connections for their “production” phone trunks.
ChannelWeb is publishing a review of five VoIP systems. While not terribly helpful as a whole, the article discusses several aspects of choosing a VoIP system. The focus is on small systems for small business, 2-40 users or so. There are some helpful comments following the article.