VoIP technology makes it easy to interconnect between systems. It is likely that many VoIP enabled companies don’t even realize that all the technology already exists to route calls across internet connections using the same numbering plan the PSTN uses.
Initiatives such as ENUM have already developed the protocols to handle this, and if you have a next-generation open-source PBX, you already have this technology.
What I’m not sure is when the tipping point will be. For any company to take advantage of what amounts to free long distance calling anywhere in the world, two things have to happen: 1) you have to be referenced in the ENUM databases, and 2) any company you wish to call has to similarly be referenced.
The technology is ready. Next we have to figure out how to get everybody on board.