A phone receiving many calls in rapid succession can be said to be "ringing off the hook". (Often cartoons will show a telephone handset literally bouncing above the ringing base unit.) This most likely led to the present use of "off the hook" as a synonym for "crazy" or "exciting".
"Off the hook", meaning "freed from responsibility or culpability" is not related to the telephone, but most likely derives from the image of a worm dislodging itself from a fishing hook.
""Off the Hook"" is sometimes used as slang, when some thing was really good, or some one had a good time
Example usage
"Last night was off the hook" as if they were saying "last night was really awesome"