AT&T SYSTEM 25 Manual page 158

An attendant can use POS BUSY to send most calls that come to the attendant console to
the other attendant, or (SLAC only) to a designated "backup" station. Only one attendant
console can be placed in Position Busy mode at a time.
DTAC
For DTAC systems, POS BUSY can only be used when there are two attendant consoles.
Placing a DTAC in the inactive (Position Busy) mode disables ringing on line appearances on
the two button columns on the far right side of the console only. All buttons on the inactive
console will continue to function normally, including the Selector Console buttons. Calls can
be originated by the inactive console. Call appearances in the first two columns of buttons on
the inactive console are not affected by Position Busy.
When a console is in inactive mode, the following calls are transferred to the active console:
Calls to FPDCs that are not signed in anywhere
Calls to unassigned DID numbers
Dial "0" calls
Calls on line appearances on the rightmost two columns of buttons that appear
at both consoles
When a console is in inactive mode, the following calls remain at the inactive terminal:
Internal calls to the inactive terminal's PDC
Calls to line appearances in the leftmost two columns of buttons
Calls to a personal line not shared by the other console
Position Busy
Position Busy 3-41