At\u00a0the first Network World Showdown\u00a0at the Voice on the Net show last\u00a0April we asked a panel of IP PBX vendors if IP telephony was ready for prime time. They assured us it was, although the debate that endued was littered with fundamental questions that exposed some raw spots.We followed that up at the Fall VON last September with\u00a0a showdown\u00a0pitting three IP PBX vendors against three IP Centrex service providers. When we asked the audience to vote for the team that told the most compelling story, the crowd gave the win to the IP Centrex folks.Now we're gearing up for Spring VON '03 and bringing the focus back to the IP PBX camp. Although this market is still young, it is mature enough that we no longer need to ask ourselves if the technology works. Instead, we're inviting some of the biggest vendors in the business to discuss what makes their products unique.As the early technology debates fade, the IP PBX suppliers increasingly are turning their energies to differentiating their wares, touting everything from innovative station alternatives to management controls and security capabilities.We'll talk about those and other differences, including call-center support, the simplicity of moving to unified messaging, and the manner in which new applications are developed.That's not to say some core technology discussion won't creep in. Questions remain about single-number portability, support for remote nodes and even the manner in which vendors meet the requirements of the Americans with Disabilities Act. We'll sample these where appropriate.We call on Alcatel, Avaya, Cisco, Mitel and Nortel to participate in this Network World Showdown, "IP PBXs: Telling the players apart," at Spring VON, from 5 to 6:30 p.m. March 31 at the San Jose Convention Center (to register, go to VON.com). They'll have until March 3 to confirm.To refresh your memory on how these Showdowns work: In the first part of the session, yours truly and co-host, Mike Hommer, manager of consulting for Miercom, which is a member of the Network World Global Test Alliance, pose questions to the individual vendors.In the second third, Hommer and I play referee, letting the vendors question each other. And finally we open it up to members of the audience.So plan to join the debate this spring, and in the mean time send along any questions you would like to see addressed.