Are you sure you mean a calling on arm? I know it was asked earlier but it's called a Calling On (CO) signal because you are calling the engine on to something i.e. usually a train. It usually works with track circuits (TS) such that if a train is standing in the section, in advance of the Home, the activation of the TS allows the pulling of the CO arm but locks the Home. When no occupation of the TS the CO is locked but the Home released.