Hi Jay,
At the moment I don't have the time to look it up. (I'm on my way home from work) Ianobi has an excellent home pbx tutorial which is probably what you want. Have a look at his posts.
Here is that post I saved it. There is also another excellent
post by ianobi on the same topic.
When I find a really good post I save them to my computer for easy reference. I have a few by ianobi.
I know this sounds like a ridiculous application,
I guess the application can't be all that ridiculous since it's already been asked. The fact you plan to use a couple 200 units doesn't change the setup from what was already laid out.