off the hook idiom
（电话听筒）未挂上
If you leave the phone off the hook, you do not put it back correctly and it will not ring.
脱离困境，脱身
If you are off the hook, you have escaped from a difficult situation.
John's agreed to go to the meeting in my place so that gets/lets me off the hook. 约翰答应代替我去开会，这样我就能脱身了。