just to respond to prostreetcamaro's comments:
-most- of the time, the voip's call quality doesn't depend on the voip provider. (of course.. there are exceptions)
the real problem is either your upstream speed isn't sufficient or if your router can't provide QoS.
now, i'll be the first to admit AT&T callvantage's customer service is better than vonages. i have friends who have both and their experiences w/ CS are different -> wait times, attitude,...