Brian, either I'm reading too much into this or the explaination is a little vague but this is what I'm getting;
You have a Main Number 354-998-9800 (let's say) and when someone calls this number, you have it go to a Ring group (800, let's say) where every phone in the place rings
You have employees that have their own extensions (hopefully in the same pattern range, 354-998-9801, 9802,9803, etc. [let's say]) and when people call these numbers they go directly to that desk and nowhere else. But, when a second call comes in on that same number, the second caller gets a busy signal? Does this happen on your MAIN number too?
If so, it sounds as if your provider has the number of channels set too low (2 maybe). If that's not it then maybe voicemail is inactive, or take a look at the extensions page (pick any extension you want) and scroll to the bottom and see if you have Alternate destinations settings for Busy, Unavailable, Unreachable (they should all read something like, 'Use System Default'). Change the BUSY setting to Ring Group <800> Main Number Ring Group.
What you describe happening sounds more like insufficient Channels (Concurrent Call Slots per Number [On DID Provider's end]) that if the DID has no place to send the call to the busy signal is their default.
Also I don't think Green is supported anymore (I could Be wrong here) I think the colors are older versions.
Hope I am not is outer space with my understanding of your problem. Good Luck.