Off Hook

The signal received at the Central Office indicating that the telephone receiver has been lifted (activated). On switched telephone networks, originating off hook prompts the Central Office to activates a dial tone on the originating end telephone receiver indicating that the Central Office switch is ready to receive a number. When the CO switch receives the full telephone number, the call is routed and a ring signal is sent to the receiving end / destination phone line.   Destination off hook ( When the telephone call is answered at the receiving end. ) At this point, both parties are connected  ( and per minute billing is activated for long distance or local toll calls ).