You will have to test this but..what if you ring the line into a single line port with an unplugged device. Place a secondary Ext key of that single line ext on the boss' desk. When the single line is called, the boss hits his 2ndary ext key and answers the call. I do not believe you can reverse transfer a call ringing on a secondary ext key. So as long as the staff does not know the single line ext, he should be good. Like I said, test it but it just may work.