Exactly this. It’s called a “hook” and when the phone is “off the hook” that’s the thing it is off of. Being off the hook means the phone is powered up and connected to the local loop. When the phone is “on hook” that means it is disconnected from the loop and awaiting the pulsed ring signal.

Desk phones have a reversible hook so that it keeps the button depressed when the phone is in the cradle but doesn’t catch when you attempt to pick it up.

On modem signals in the old days, the + was equivalent to “flashing” the hook, or quickly disconnecting and reconnecting to the loop, and the AT command H1 told the modem to go “on hook” while H0 told it to go “off hook”.

Back before the DTMF network, when everyone used pulse modulated phones, the “pulses” were caused by going in and off hook in a specific pattern. You could actually make a phone call from a rotary payphone by flashing the hook in the pattern that mimicked the rotary dial pulsing the line as it rotated back to home position.

In the really old days, the hand crank served much the same purpose, but actually supplied electricity to the local loop; when the phone was on hook (which was a big metal thing the earpiece sat in) someone else turning the crank would make all the phones on the loop ring; you picked up if the ring matched the number of rings for your extension.