I think it depends on who you are using for VoIP, personally. This statement below sounds like a blanket generalization to me.

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.

Time Warner Cable and MCI Inc.’s UUNET business service scored highest on reliability. Keynote found little variance in audio clarity among the broadband providers,

The study was based on 154,000 calls placed in May and June using six leading Internet phone services, or 22,000 each, and seven providers of high-speed Internet connections. [Read the rest]