Comcast has been signing up a ridiculous number of VoIP customers each quarter (half a million in Q3), and has very quickly become the nation's fourth largest phone carrier. Not only are they the biggest VoIP provider, they offer the highest quality calls, according to a new study (pdf) by Keynote Competitive Research. The study compared AT&T landline service or POTS (plain old telephone service) with six broadband VoIP providers (AT&T CallVantage, EarthLink trueVoice, Lingo, Packet8, Verizon VoiceWing and Vonage) and two cable voice service providers (Time Warner Digital Phone and Comcast Digital Voice).

Carriers were ranked based on two categories: Reliability and Audio Quality. Service availability, call completion rates, average answer time, and dropped audio performance factors all contributed to a carrier's reliability ranking.

The study concluded that AT&T landline service (996), Time Warner Cable Digital Phone (925) and Verizon Voicewing (872) ranked highest in service reliability. Comcast Digital Voice (901), Verizon Voicewing (609) and AT&T Landline (506) ranked highest in audio quality. That said, it looks like the audio quality among most competitors (particularly those running on non-managed networks) didn't pose much of a competitive challenge to the big players.

"We've been conducting VoIP studies for more than three years and have seen significant trends develop, such as serious disparity in reliability and audio performance between VoIP services on managed networks and those that rely on existing broadband home network connections," said Ken Harker, senior consultant at Keynote. "While conditions are dramatically improved for Comcast Digital Voice and Verizon VoiceWing customers, the best that other consumers can hope for is merely tolerable audio quality."

Think having your service qualified as "merely tolerable" does much to help already struggling independent VoIP operators like Vonage? At least carriers were consistently merely tolerable -- according to Keynote, only one of the providers in the study failed to provide dial tone 99.9% of the time or better (you'd have to pay for the study to find out who). According to the firm, audio quality is about the same as it was for carriers last year, while overall reliability is improving.