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