Avaya NN43113-101 - Manual (Page 278)

Additional features
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but only one call can be active at a time. However, other clients that are
part of the same MADN line can join the active call by going off hook.
You can prevent others from joining the call by using the MADN feature to
make the call private. To make the call private you dial a special code
(Vertical Service Code) so that the call server knows that the call is
private. To dial a Vertical Service Code on an IP Deskphone, you must
first initiate a new call by pressing the
Action context-sensitive soft key,
select New Call/Conference, and then dial the Vertical Service Code that
is provided by your system administrator or service provider.
Note: The MADN feature is only available when you connect to a call
server that supports it. Your system administrator manages the
MADN feature and enables the MADN feature on your IP Deskphone
if required.
When multiple IP Deskphones are configured with a MADN SCA feature,
they share one single Directory Number (DN) that is available to the user.
An incoming call to this DN or to any SIP address of the users of the
MADN SCA group causes all the IP Deskphones to ring. Also, any user of
the MADN SCA group can join a call initiated from any of the IP
Deskphones of that group.
Any user that joins a call with the MADN SCA feature can put the call on
hold and prevent other users within the MADN SCA group from joining.