On- and off-hook

In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called "seizing the line". "Off-hook" originally referred to the condition that prevailed when telephones had a separate earpiece ("receiver"), which hung from its switchhook until the user initiated a telephone call by removing it. When off hook the weight of the receiver no longer depresses the spring-loaded switchhook, thereby connecting the instrument to the telephone line.
Off-hook.
The term off-hook has the following meanings:
On an ordinary two-wire telephone line, off-hook status is communicated to the telephone exchange by a resistance short across the pair. When an off-hook condition persists without dialing, for example because the handset has fallen off or the cable has been flooded, it is treated as a "permanent loop" or permanent signal.
The act of "going off-hook" is also referred to as "seizing" the line or channel.
On-hook.
The term on-hook has the following meanings: 
The act of "going on-hook" is also referred to as "releasing the line" or "channel", and may initiate the process of clearing.