Question: Under what circumstances is a mobile subscriber considered busy in a Public Land Mobile Network (PLMN)?
Option 1: Only when the user manually activates 'Do Not Disturb' mode on their device.
Option 2: Exclusively due to network congestion, leading to resource unavailability.
Option 3: If the subscriber is engaged in a data session with no voice call capability.
Option 4: When either a 'Network Determined User Busy' or a 'User Determined User Busy' condition occurs.
Explanation: This answer correctly reflects the principle that a mobile subscriber is considered busy under two specific conditions: when the network determines the user is busy or when the user's actions result in a busy status. This understanding is crucial for managing call forwarding services and other supplementary features in telecom networks.