Your after-work time belongs to you only. Your boss has no right to contact you outside work hours; so you don't have to be available.
Please note: The general rule mentioned above does not apply if being on stand-by is a part of your company's work routine and you have been appointed for a position requiring on-call duty. If that is the case, you must, of course, be reachable. On-call duty ("Rufbereitschaft") means that you don't have to be at work (physically), but you have to be available at all times to start work, if necessary.