I don't know if this is a configuration issue, if calls are being forwarded (thus tying up an incoming and outgoing line) or something else that would account for this. It would be nice to graph how many of our lines were in use during a time period.
My assumption is that an extension to extension call shouldn't use one of the main lines, right? Up until now, everyone was in the same office so we chatted in person. Now with two locations there are many internal communications being done between extensions.