In progress
Definition 1 of 1
Pro Tip 1/3Current Action
Use 'in progress' to indicate that an action is currently happening and is not yet complete. 
The meeting is in progress, so please wait outside.
Pro Tip 2/3Unavailability
'In progress' implies that someone or something is not available because they are busy with an ongoing task. 
The bathroom is unavailable; cleaning is in progress.
Pro Tip 3/3No Interruption
Use this phrase to politely tell people not to interrupt or to inform them that a delay is due to ongoing work. 
Your call is important, but all lines are in progress.