Slate has a decent review of various VOIP services. They like AT&T CallVantage, but it sounds like you may want to wait a bit yet based on this opinion of VOIP as a whole:
Reliability also is not always up to land-line snuff. Sometimes there’s no dial tone, your outgoing calls don’t go through, or the other party can’t hear you. I also have a nagging suspicion that I am missing important calls, a fear stoked by scattered complaints like, “Your Internet phone sucks” and, “Why does your damned phone never pick up?”
Sounds like our cellphone now, so we’ll just stick with that.