AT&T Partner II Installation And Use Manual page 44

Each telephone is connected (either directly or through a wall jack) to an
extension jack on the equipment controller. Each extension jack on the
controller is identified by an extension jack number. Extension jack
numbers run from 10 through 33 (if you have the maximum number of
extensions that can be used with your premises equipment).
Line jack numbers and extension jack numbers are required for many of the
programming procedures that you use to customize your premises equipment.
Centrex Ext. 1000
Figure 3-1. Centrex and Equipment Line Numbers
Figure 3-1 shows line jack numbers and extension jack numbers in relation to
telephone numbers and Centrex extension numbers in a very small
(three-telephone) system. The three lines in this example (555-1000, 555-1001,
and 555-1002) use the last four digits of the phone number as the Centrex
extension number. The first line has Centrex extension number 1000; it is
connected to the first line jack on the equipment controller (line jack 01), and
serves as the primary line for the phone connected to extension jack 10. The
other two lines are connected to line jacks 02 and 03, and provide primary lines
for the phones connected to extension jacks 11 and 12, respectively.
NOTE:
Figure 3-1 shows only primary line assignments. By default, the equipment
controller assigns all lines to the phone connected to extension jack 10, but only
the first line is primary.
3-4 Using the Equipment
Equipment Controller
555-1000
555-1001
555-1002
Line Jack 01
Ext. Jack 10
Centrex Ext. 1001
Default Primary
Line Assignments
Line Jack 02
Ext. Jack 11
Centrex Ext. 1002
Line Jack 03
Ext. Jack 12