AT&T MERLIN LEGEND System Reference Manual page 125

Features
Single-Line Telephones
Mode Differences
Behind Switch
2-76 Conference
To add another person, the user presses the Conference button again. The
green LED next to the line buttons flash, but the participants can converse. The
originator can continue to add participants by pressing the Conference button,
selecting a line or dialing the number, and pressing the Conference button until
up to two outside lines and three extensions (including the conference
Originator) are in the conference. The user can converse privately with each
participant before pressing the Conference button to join other participants.
Calls to busy numbers cannot be added to a conference. If the user reaches a
busy number, he or she can press any of the line buttons associated with the
conference call to disconnect the call to the busy number before continuing to
add participants.
To selectively drop a participant the multiline telephone user presses the Drop
button followed by the line button for the participant to be dropped. To leave the
conference call temporarily without disconnecting the call, the user presses the
Hold button. To rejoin a held conference call, the user presses any line button
representing a conference participant. To end the conference, the user hangs
up and all participants are disconnected.
A single-line telephone user sets up a conference call by pressing and releasing
the Recall button or switchhook after receiving a call or dialing the first outside
number or extension. The participant automatically goes on hold (an outside
participant hears Music-on-Hold if it is programmed, and an inside participant
hears nothing). The user then dials the other number and presses and releases
the Recall button or switchhook again. All participants are connected on the
conference call. A total of three participants can be included on a conference
call originated from a single-line telephone. The user can converse privately with
each participant before pressing the Conference button to join other
participants.
Calls to busy numbers cannot be added to a conference. If the user reaches a
busy number, the user can press and release the Recall button or switchhook to
drop the outside line.
A single-line telephone user can drop the most recently added participant from
the conference by pressing and releasing the Recall button or switchhook.
Note: If a single-line telephone with a timed disconnect is used (for example
AT&T models 2500YMGK-PEC 3178-NHL and 2500MMGK-PEC 3101-ETR),
pressing the switchhook automatically disconnects the call. With this type of
telephone, the Recall button must be used instead of the switchhook to add a
conference participant or release the most recently added participant.
In the Behind Switch mode, the fixed Conference button on multiline telephones
activates Conference from the host system. The dial codes for the host system
for Conference and Drop must be system programmed. A multiline telephone
user can program a Conference or Drop button to use the Conference feature
as described above. A single-line telephone user cannot use the Conference
feature in the Behind Switch mode.