I'll link you to the lead page for each critique. There are examinations of services provided by AT&T CallVantage, Broadvox,Lingo,Verizon VoiceWing, VoicePulse, and, of course, Vonage. There's also a section on what PC Magazine calls "Sometimes-Free Internet Services" - like Skype.
The editors voted for AT&T CallVantage as the best service. The service's conference bridge capabilities and house-call availabilities were seen as plusses.
As one who has participated in many conference calls, and who haspaid out more than pennies fortechs from across town and across the country to fix things in my office, "I say good on you, AT&T."
The report does have some omissions and weaknesses.
Second, I would have loved to have seen some sort of lab test comparison about the call quality for test calls using each provider.