Looks like CSCee46014 matches what you describe...
Symptom:
In the System Administrator, for a subscriber, in the messaging page, when
trying to uncheck the "Use MWI for message notification box" in order to disable MWI for a user, the setting does not take hold when the 'Save' button is pressed.
Conditions:
This problem occurs in Cisco Unity 4.0(4). The problem is fixed in Unity 4.0(5).
Workaround:
First delete all existing MWI extensions for the user, only then can you uncheck "Use MWI for message notification" and have the change stick in order to disable MWI for that user.