Just to echo what has been said...get a dealer/cabler involved so that they can do it right the first time. I can't tell you how many times business or even electricians have wired for phones and network incorrectly and we have to come in and fix it later. We even recently came in to help a person whose husband wired the office (she was a realtor too...interesting, no?)
Also, I would echo Grider...VOIP does have some specific niches, but to act as a PBX for a small office is not one of them...unless you are a remote office running off of another main office's system (and even then, you may be in for a lot of service glitches, depending on what comes into you building).
Bottom line, working with a dealer/cabler is an investment WELL SPENT. You have a person who will know your wiring, needs, and preferences. This will save a LOT of time when you need an extension added, moved, etc. because the person doesn't have to play detective as much figuring out what goes where, or having to run new cable because it wasn't done right the first time. (remember that service is charged by time by the way).
I have seen this sort of thing happen time and time again (and I know I'm not alone in this). We are not trying to be difficult, we really are trying to help.
