Automatic call distributor

In telecommunication, an automatic call distributor (ACD) is a device that distributes incoming calls to a specific group of terminals.

Note: If the number of active calls is less than the number of terminals, the next call will be routed to the terminal that has been in the idle state the longest. If all terminals are busy, the incoming calls are held in a first-in-first-out queue until a terminal becomes available.

Source: from Federal Standard 1037C and from MIL-STD-188