I have a customer that is having some trouble with his pref line out. When he picks up the handset to go off hook the display says "Not Valid". Now his phone is pref line out to intercom, if I change it to pref line to "idle" then it will grab an outside line fine. He only gets the "Not Valid" error when his pref line is intercom. I am by no means a Panasonic tech and have limited experience with the system. If there are questions I will do my best to gather the information for you. Thanks for any help.