Stealth Communications has announced the ENUM registry, which will allow VoIP providers to complete calls without going through the public switched telephone network (PSTN). When a VoIP called completed, it usually goes over the "old" telephone network at least part of the way. In turn, the VoIP provider has to pay an access fee to the network owner (e.g. Verizon, SBC, Qwest, etc).
The ENUM registery is a free service that links a VoIP telephone number with the IP address where the phone is plugged in, so the VoIP provider can simply look up the called number in the registry and send the voice packets straight to the IP address of the receiving phone.
This sounds simple, and it is. But it is critically important, because it provides a way to build a "pure" VoIP global voice system without ever using the old switched system. It's the last piece of the VoIP puzzle.