AT&T System 25 - Manual (Page 36)

Exclusion
This feature prevents others from listening in on a shared line.
If you are on a call and activate this feature, it will automatically
disconnect all other inside lines. To include selected inside people
on your call, press before adding them to the call.
Activating Exclusion
1 Answer or place a call.
2 Press
The light next to this button turns on,
NOTE: The Exclusion feature goes off automatically at the
end of your call.