In my very unscientific tests here in the home lab, I have noticed a significant difference in call quality between Vonage and CallVantage.
I’ve made calls at similar times with similar loads on both my internal network as well as traffic flowing in and out via iPodder and Bit Torrent. MIleage may vary for you, but I have yet to hear or experience any crackling, skipping or stuttering with CallVantage and have heard my share of issues including unusable connections on Vonage.
Price is one issue and Vonage is certainly leading there, but CallVantage delivers the better voice experience (and still saves you mucho dinero) which is the real issue with phone service. It needs to just work.