On- and off-hook

In telephony, the term off-hook has the following meanings:

The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece (i.e., receiver) which was removed from its switchhook. The weight of the receiver no longer depresses the spring loaded switchhook thereby connecting the instrument to the telephone line.
One of two possible signaling states, such as tone or no tone and ground connection versus battery connection. Note that if off-hook pertains to one state, on-hook pertains to the other.
The active state, i.e., a closed loop of a subscriber line or PBX user loop.
An operating state of a communications link in which data transmission is enabled either for (a) voice or data communications or (b) network signaling.