Precondition: You have dialed an internal number and hear a busy signal. It is important that you reach the called party. 

Wait (approx. 5 seconds) until "Camp-on" appears on the display and the busy tone is followed by the ring tone. 

The called party can then respond  

The called party can prevent automatic call waiting .  If this feature is configured (contact System Support), you will hear the ring tone and the message "Camp-on" is immediately displayed.