Question: What happens when a User Equipment (UE) user, alerted by an incoming call, enters '0 SEND' in response to the alert?
Option 1: The call is automatically answered with a voice message.
Option 2: The UE initiates a call back to the caller after a short delay.
Option 3: The incoming call is put on hold for the user to decide later.
Option 4: User Determined User Busy (UDUB) is set, potentially invoking call forwarding on busy or presenting 'BUSY' to the caller.
Explanation: When a UE user enters '0 SEND' upon being alerted of an incoming call, it triggers the User Determined User Busy (UDUB) feature. This action does not automatically answer, put the call on hold, or initiate a callback. Instead, setting UDUB either activates call forwarding if the service is enabled and operational for busy situations or signals to the calling party that the user is currently busy, thus presenting 'BUSY'.