The Advanced MWI property page allows you to configure the line that will be used to perform MWI notifications for this mailbox.
As noted on the dialog, this is an advanced setting that should only be changed if necessary. For the most part, you would only need to configure this setting if your PBX is an Avaya ACM or Definity. and you have enabled more than on port to perform MWI notifications in the profile settings.
Use the selection box to assign a specific line to perform MWI notifications for this mailbox. Or select Default to have the system select a line for each notification at the time it occurs; this is how the system normally operates.
Alternatively, you can click Randomize to have the system randomly assign a line. It will use an internal algorithm to randomly select one of the lines configured for MWI notifications for the profile assigned to this mailbox. The selected line will be auto-selected in the selection box and, if you click Apply or OK, that line will be used for all MWI notifications for this mailbox.
To save your selection, click Apply or OK. To cancel without saving any changes, click Cancel.