Our 3Com NBX system is great quality and extremely reliable. But it runs on Layer 2 (ethernet). We have the IP license and have used it over the net on a VPN with pretty crappy results.

On the other hand, I've had marvelous experience with Skype. In fact, one of these days, I'll probably drop Voicepulse (which I've been pretty pleased with) and just get a skype-in number.