If you don't watch closely enough you could miss it: The voice-over-IP revolution that many were hailing just three years ago is finally happening — just not in the same way or at the same speed that most anticipated.

Instead of the softswitch-centric access networks that were suppose to herald a new generation of converged voice and data services (and displace every circuit switch in existence), the upheaval is more muted, more like a sub-sonic wave that can't be heard by the human ear but could have a lasting impact well into the future.

Pinning down exactly how VoIP is rolling out and the direction it will ultimately take has become a guessing game in which every answer not only is right, but also can be backed up by some kind of carrier push. Far from being pigeonholed into a neat category, VoIP now is more scattershot, with few carriers and vendors agreeing on even simple definitions of the technology.

Verizon Communications, for instance, already sends about 11.5 million calls over an IP portion of its network that resides behind its access switches. Sprint, meanwhile, will have deployed its first softswitch in an end office by the time the convention floor opens up for this year's Supercomm in Atlanta. Both say they are using VoIP.

“The year of carrier deployment is now,” said Al Safarikas, vice president of wireline marketing for Nortel Networks. “The question is the network architecture and the part of the network they choose to packetize.”

Add to the mix the fact that many enterprises have taken the initiative to implement VoIP without carrier involvement, and an even more jumbled view of the market emerges. By the end of the year, more than 20% of U.S. enterprises will implement IP telephony at more than five sites, according to research firm InfoTech.

In truth, the market for VoIP is moving forward at what would normally be considered a fairly good clip for any technology. The two most common applications, IP PBXs and IP Centrex, have made significant inroads in part because IP phone prices have fallen into the range of their analog/digital counterparts, according to a recent report from research firm Probe Group.

Still, given the outsized expectations that saddled the market, there is often a perception that VoIP has failed. “Sometimes we hear that voice over IP is dead,” said Neal Shact, CEO of Communitech and chairman of the newly created VoIP Council. “But we know that the economics are so compelling it's going to happen anyway.”

Indeed, many of the vendors at this year's Supercomm are expected to emphasize applications like voice over VPN and videoconferencing made possible by softswitches.

“With video included, it's not just a financial bundle,” said Safarikas. “It becomes a functional bundle.”