Landlines still better (for now)
Internet-based telephone services are still very inferior to traditional phone connections in reliability and sound quality, according to an extensive study that judged Vonage and AT&T CallVantage best among the top providers.
Keynote Systems Inc., known for measuring the performance of popular Web sites, also found that the reliability of Voice-over-Internet phone service is significantly affected by the provider of the high-speed Internet line used to dial a call ...
Disruptions in the ability to make or receive a call was a key problem, the study found. Keynote said service availability ranged from 99.4 percent of the time to as low as 94.8 percent, meaning that for customers of the worst performers, one out every 20 call attempts failed.
Whole article here