An early telephone handset hung from a hook that activated a switch.When the telephone was not in use, the handset hung on the hook, or was on-hook.When the telephone was in use, the handset was off of the hook, or off-hook. When the user lifted the handset off the hook, a spring lifted the hook, which closed a switch and closed a circuit, drawing current from the central office (CO).The term now refers to the mechanical buttons or plungers that are mounted in the cradle of a telephone set, but the process remains essentially the same. Synonymous with hook switch. See also off-hook and on-hook.