I think I recall seeing a post here at one point that stated they turn it off every now and then for training purposes as to allow the dispatchers to keep their skills in case of the systems failure. It would also be plausible that they turn it off during periods of heavy or potentially heavy call volume as I noticed a few times before that it some times takes a while to hear the runs go out. Last night would have been a perfect night due to the recent rash of arson...
Just some thoughts.