AT&T MERLIN LEGEND Analog Direct-Line Console Operator's Manual page 29

Understanding Your Equipment
The Shift buttons (1, 2, and 3) are used with DSS buttons for
reaching the extension numbers assigned on your DSS. The
"Shift" refers to a range of numbers on the DSS. Each DSS
button can have three numbers on it (the example shows 10,
50, 90 on the first button). By pressing different Shift buttons
(1, 2, or 3), you can access alternate ranges of numbers. For
example:
Shift 1 is used to reach the first number on each DSS
button. This button allows you to reach phone numbers
10-49.
Shift 2 is used to reach the second number on each DSS
button. This button allows you to reach extension numbers
50-89.
Shift 3 is used to reach the third number on each DSS
button. This button allows you to reach extension numbers
90-129.
Note: After you press a Shift button, press a DSS button to
dial a co-worker's extension.
1-16 Line and DSS Buttons