The Automatic Callback (ACB) service allows you to be notified when the previously called busy party becomes idle and is connected without having to redial the phone number.
When you call another party and the call cannot be connected because the line is busy, you are prompted to make an automatic callback request (for example, “The line you are calling is busy. Press 1 if you would like to be notified when the line becomes available”).
If you select Automatic Callback, the system monitors the busy party and automatically establishes a call when the busy party becomes available.
You can enable or disable Automatic Callback in the Preferences and Incoming Calls tab.