DIRECT LINE APPEARENCE -
If you have a button on your phone that is an OUTSIDE LINE, then that is a DIRECT LINE APPEARENCE.
If your system is SQUARE, then you have DLA's for sure.
If it is set for HYBRID-PBX, you only have DLA's if sets were programmed with them.
If phone A has the same DLA's of phone B, then phone a can access what ever line phone B is using. UNLESS, PHONE B has ACTIVATED PRIVACY.
To insure that Service Observing has been done away with, go to SPM Admin and print out Service Observiing. That will tell you right away.
To find out if LINES ARE ASSIGNED, you'd have to do a PRINT ALL, which can take HOURS, and look at individual extensions. (Unless you know you are in KEY MODE.)