AT&T Merlin II Release 3 System Manual page 116

By default, the system assigns
the first three lines to 7406D
phones in the order indicated
by the numbers. If you remove
those lines and assign others
to the 7406D phone, the new
lines are assigned to buttons
in the order shown.
FIGURE 3-12 The order in which lines are assigned to buttons on digital phones
in a square configuration.
MERLIN II System Features 3-87