HS-1 Switch hook Also called a "hook switch," it is the control mechanism that answers and hangs up a call on a telephone. When you place the handset in the telephone cradle, it depresses the switch hook"s button and hangs up (puts the phone "on hook").

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.