O.K. as long as there is a vacant key available on the phone for a new call to come in on it is not "Busy" The phone would only be considered busy by the system if there is no way for a call to be presented to the set. If you think of it this way. Can the user see and answer another call even if they are on the phone? If the answer is yes then their phone isn't Busy, only not answered. If you removed the second call key and the trunk key and only had one path for calls to be answered on and it was in use, then it would be Busy.
