For our fourth Network World Showdown at the\u00a0Voice on the Network conference\u00a0- this one Sept. 23 in Boston - we challenged a handful of IP PBX vendors and IP Centrex service providers to come mix it up in a presidential-style debate. All have accepted.Our lineup will include:\u00a0Cisco's Bill King, manager of technical marketing;\u00a0Avaya's Danzil Samuels, vice president of the service provider division;\u00a0Nortel's Tony Pereira, director of Enterprise Solutions Marketing;\u00a0GoBeam's Jeff Stern, executive vice president and co-founder; and\u00a0M5 Networks' Phillip Kim, CFO and director of research and development.The first three are household names, but GoBeam and M5 might need some introduction. GoBeam offers what it calls a virtual PBX service through a string of local dealers, and wholesales service through some big names like Verizon. M5 offers what it calls a Managed Telecom Service, primarily in the New York area.We won't pit the Centrex camp against the customer premises equipment (CPE) suppliers as\u00a0we did last October at VON, but simply treat all comers as potential solution providers. Even though IP Centrex is still only available on a limited basis, it is gaining momentum and should be considered by enterprise customers as a viable alternative.One of the beauties of voice over IP (VoIP), after all, is it doesn't much matter where the call controller is on the IP network. That means telephone companies can host the servers and offer managed services that rival the benefits of CPE-based offerings.Besides relieving customers of basic management responsibilities, hosted services offer a better disaster\/survivability story because: 1) the controller is off-site; and 2) telco central offices have about as many built-in safety features as you can get. What's more, with IP Centrex, customers can wipe their hands of upgrade hassles and version control problems.IP PBXs offer their own advantages, of course - you can migrate more gradually and IP PBXs can be used with a range of service providers - but the point is that VoIP levels the playing field. It is time to examine both options when an upgrade is in order.As in previous VON Showdowns, yours truly and co-host Mike Hommer, manager of consulting for Miercom, will open the session by posing questions to individual vendors. Then Hommer and I will play referee, letting vendors question each other, and then we'll open it up for questions from the audience.Hope to see you at the show: Sept. 22-25 at the Hynes Convention Center in Boston.