Hi , I wonder if someone could help me with this ? I am unfamiliar with the above system . My customer has 3 existing CO lines ,and wishes to add a 4th line . Looking at the system , it looks like there was a 4th line at one time , but its no longer there ,although the patch cord for lines 3 & 4 is obviously there . Any advise on the programing to get a 4th line working , and also the incoming ringing sequence would be appreciated / thanks David Welsh .