AT&T 1040 Quick Reference Manual page 12

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INTERCOM FROM 
Intercom
Answer an intercom call
When you receive an intercom call, you will hear a
repeating double-ring pattern and your screen will
display INTERCOM FROM with the extension number
of the caller. Answer the intercom call by lifting the
handset, or by pressing [INTERCOM], [ SPEAKER]
or [HEADSET] to take the call hands-free.
NOTE: If you press [INTERCOM] to answer the
call, the phone will automatically use the mode
(speakerphone or headset) you programmed as
the automatic mode (see the AUTOMATIC MODE
section of your installation guide).
End an intercom or page call
To end the intercom or page call, press [INTERCOM].
-OR-
Hang up or press [ SPEAKER] or [HEADSET]
again.