If you leave the phone off the hook, you do not put it back correctly and it will not ring.(电话听筒)未挂上
If you are off the hook, you have escaped from a difficult situation.脱离困境,脱身
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。