Vertical Communications DSU and DSU II - Manual (Page 46)

2.15.1 Accessing The Button Levels
The IB24X and IB48X DSS/BLF consoles have four levels of buttons,
quadrupling the consoles’ button capacity. The four buttons at the
bottom of each console allow you to select which button level is
currently active. To change button levels, press one of the buttons at the
bottom of the console. The leftmost button activates level one, the next
button activates level two, and so forth. A light beside the appropriate
button lights to indicate which level is currently active. You can use one
level on the IB48X and two levels on the IB24X.
ib48x
Level access buttons
IB48X DSS/BLF Console
ib24x
Level access buttons
IB24X DSS/BLF Console
Programming The Attendant Features GCA70–245
2–26 LCD Speakerphone Reference Manual