The do not disturb feature allows users to set a condition on their handset phones which would automatically reject a call.
This is somewhat similar to call forwarding, except that any incoming call will terminate to the users 'busy' voicemail (regular unanswered calls terminate to the 'unavailable' voicemail). This feature is useful when a user is occupied with an important task and cannot afford to take a call (e.g. important meeting, important task, eating lunch, etc.).