AT&T System 25 - Manual (Page 43)

Inspect
If your telephone has an attached display, you can use the
button to do the following:
Identifying Calls on Hold
The display is the same as the one shown when the call first
came in, as in these examples:
329 Tango, S
or
OUTSIDE
Identify Programmed Numbers
You can identify the number that is stored on Flexible DSS and
Repertory Dialing buttons, and Last Number Dialed. For
example:
If no number is currently stored, you will see the feature name
instead of numbers on your display:
Determine Status of Outside Lines
The display tells you the number of busy lines out of the total
number of lines in a pool. In the following example, five of the
12 trunks in the pool of trunks are busy.
005 OF 012 BUSY
40