OK - here's an odd one. I have a number built in 3 physical places and 2 key appearances. 2 of the phones are 3904 sets, and the other is a 500 analog. (Key appearances are 3904's) The thing keeps toggling on and off hook on either of the sets. I tried swapping out the phones one at a time. No change. I tried to disable the TN's one at a time and no change? Any thoughts?