First off, I don't know where you got the quarter second latency figure, since even before WB scrubbed their online documentation, even they only claimed a half second latency. I can say without equivocation, as both a former customer and business partner, the best latency figures I saw (prior to DAMA) on both the front and back end was 600ms (.6 seconds) with an average closer to 700ms. After DAMA was implemented, you will not see latency on the WB system under 1 second, with averages (on well installed systems on non-overloaded beams) closer to 1200-1400ms (1.2-1.4 seconds). This is between the modem and the gateway, and does not even factor in subsequent geographical traffic.
Bottom line: WB is NOT suitable for voip. They have even changed their online documentation to state that they do not support voip. They are great for browsing, e-mail, and downloading (carefully, as they do have a fairly stringent FAP). WB is horrible for ANY real-time application such as voip or gaming. I have heard that Hughes is better in terms of latency, but I have no firsthand knowledge one way or the other. I personally switched to a local wireless ISP (WISP) that utilizes Motorola Canopy hardware, which is growing fast, but may not be available in your area. It not only supports voip, many ISP's actually prioritize voip traffic.