Lots more good info here. Allow me to take my own "cut to the chase"
Yes, this is true, especially in large shops where the FLU (forklift upgrade) is not a viable option. Bandwidth permitting, cross country IP trunking, as we are doing today with Mitel 3300's running between Houston, Boston, Nashville and Philadelphia, is another of these unique VOIP features that can provide rapid ROI, often in less than a year (eliminating multiple long-haul voice T1s by leveraging underutilized data bandwidth). It's also a way-cool way to cheaply establish remote extensions literally anywhere at a moment's notice, i.e., Houston sales agents working from offices in Boston but with Houston D-I-D numbers. We of course can do it from home too, but our Sr. mgmt still has misgivings about "maintaining control" over the workforce, so there won't be any telecommuting for a while.
I honestly think had the Cisco VAR proposed this (IP trunking) to us, instead of attempting to hollow out the account and start us down the road toward *their* goal of an eventual total FLU, we might have been more receptive. As it was, their concept of migration to a totally PC-based server farm was an instant turn-off complete with red flags, sirens and lights! I honestly think we'd consider using Semifore between the rooftops before we'd consider putting the corporate office phone system on any PC platform, whether Microsoft or Linux.
I've mentioned Linux twice now in recent postings because it has been brought to my attention that Cisco is considering migrating their Call Manager product to Linux as a way of addressing customer concerns with the historically unstable and unreliable Microsoft OS's. While Linux would be an improvement in that area, it still doesn't address the frailties inherent with most PC hardware in general. True there are "industrial strength" PCs designed for continuous commercial service, but those machines are expensive. If I'm going to spend that much gelt on PC hardware (that will in all likelihood still become obsolete in 3-4 years), I'm going to buy a full blown PBX, not some damned kit of components and software applications. The server farm concept isn't going to fly. That dog won't hunt.