If this feature is configured (contact System Support), you can reserve a busy trunk for your own use.
When the trunk is free, you receive a call and a message appears on the display. 

Precondition: The message "US:Currently busy UK:busy at the moment" appears on your screen. 

Confirm. 

When the reserved trunk becomes free:

Your telephone rings. The display shows " Trunk is free". 

Lift the handset. You hear the CO dial tone. 

Enter the number of the external station.