AT&T MERLIN LEGEND System Reference Manual page 328

Features
Multi-Function Module
Personal Lines
Queued Call Console
Recall
Signaling
System Access
The use of Privacy is not recommended for MFMs because the user does not
have an LED to indicate when the feature is active.
When an individual personal line is assigned to more than one telephone, users
cannot select the Personal Line button to join a call if Privacy has been
activated.
A QCC system operator cannot use Privacy.
The single-line telephone user with a Recall button must use the Recall button to
activate or cancel Privacy during a call.
Users can program and use signaling to get through to co-workers who have
activated Privacy.
In the Hybrid/PBX mode, if Privacy is activated on the principal station for a
System Access button or by another user with a Shared System Access button,
other users, including the principal station, cannot join the call. If Privacy is
activated after another user joins the call, it does not affect that person who
joined the call, but no other users with a Shared System Access button can join
the call.
Privacy 2-279