Allows users tracking the condition of the other telephone or the subscriber, for example, there will appear a signal on the screen of the telephone device informing that the subscriber is on the line and is temporarily unavailable.
IDC projected over 9 billion minutes of voice traffic to travel over worldwide packet networks in 2000, exceeding 135 billion minutes in 2004. Service revenue is projected at 1.6 billion in 2000 and 18.7 billion in 2004.