UCP Call Waiting
This applies to version 14 of your PBX GUI
The Call Waiting widget used to be part of the Device Settings page. In UCP 14+ it's now a self contained widget.
The User Management Module controls which Call Waiting controls a user will be able to add as widgets in UCP.
While on the phone will this user be notified of an incoming call and is able to place the first call on hold while answering the second.