Surprised to hear of your experience of Sipgate. I've been using them for over 5 years now, admittedly only as a home user, and not had too shoddy an experience. And I have tried others (localphone being probably by 2nd fave) but have stayed with Sipgate.
I've always thought VOIP to be a somewhat, um, erratic performer regardless of provider but it is much more reliable these days than when I first started out on it.