When the telephone handset or receiver is off the cradle; the circuit is closed (active) and ready to communicate information.
The active state of a customer telephone circuit. The term is derived from old telephone sets where the receiver, when in use, was lifted from a hook attached to a switch. The opposite condition is on-hook.
This refers to an open phone line. The term originated with early phones in plain old telephone service (POTS). The receiver of the telephone (handset) was lifted from a hook (now more commonly called the hookswitch) in order to signal the central office (CO) for an open line.