What is the driving force behind VOIP?

Open Source! according to Mark Spencer, driving force behind one of the most popular open source VOIP project, Asterisk.
Asterisk is a PBX with its own protocol and interconnect system. But it interoperates with almost all the protocols available today.
n addition to traditional PBX services, Asterisk provides three-way calling, conferencing, voicemail with directory and caller ID. It's also a gateway for VoIP interoperability, handling SIP , H.323 and Media Gateway Control Protocol specifications.
"Open Source is really driving innovation," Spencer told the audience of VON (voice over net). He described the IAX Protocol as NAT (define) transparent (which is huge issue for other protocols) and is an efficient transport that can triple calls per megabit versus SIP G.729.

Spencer also discussed DUNDi, an open source peer-to-peer routing protocol. DUNDi brings the P2P model to call routing and even involves a new type of agreement for carriers for peering that's modeled on the GPL, called the GPA (General Peering Agreement).
Spencer told representatives of Canada's telecom industry that "open" concepts have a historical precedent as old as Isaac Newton. The 28-year old Spencer put up a famous Newton quote, "If I have been able to see further, it was only because I stood on the shoulders of giants," to help explain how open source uses the collective power of many to spur innovation.
In Spencer's view, manufacturers have only a few options in the new competitive environment. They can try and stay ahead of where open source is or embrace open source solutions through either productizing open source solutions or integrating open source tools into existing products.

"Or you could become a disruptive open source leader yourself," Spencer said. "Open source and business can co-exist. Open source and the GPL are about the customer first not the vendor, so you have to adapt to those things."