3Com has a very interesting and useful way of psoitioning their VoIP equipment - they call it Voice Boundry Routing. You can download the whitepaper, but here is the piece that summarizes one of the key points of enterprise VoIP peering
With an enterprise-wide deployment, organizationscan greatly improve the reliability of
their telephone service for both customers and internal users because there are many
available paths known by the network of call controllers, as compared to only those
paths known by a single call controller. This constantly updating network-mode of call
path determination is a major improvement in call routing services. At any point in time,
the network of call controllers knows the greatest set of possible call paths. With
knowledge of so many available alternatives, network outages can be easily bypassed.
By KNOWING "network outages can be easily bypassed". This implies that many enterprises do not know their call path, or simply the path that their "network" takes from any one point to another outside of their office locations. Maybe they have a high-level network diagram, but not a true physical description. In most cases enterprises that are not going to procure their own fiber will never know that level of detail, so the application layer approach of knowing the voice nodes and paths is the next best thing.
Knowledge is Power! It is amazing what an Ethernet/IP/SIP network can do when you know how to do it. The 3Com approach to SIP is very logical and walks the enterprise through the steps. They stop short of fully explaining the possibilities of inter-enterprise VoIP Peering which I suppose they might call Voice Cross-Boundry Routing, but that's ok. One step at a time. Let the enterprise build thier own VoIP islands first. Once those merits have shown themselves the next step of mass-enterprise peering will be logical and incremental.