You know I worked with a property in Houston for a couple of years. the manager had us doing all kind of things to route calls different places, etc. He kept telling me he just couldn't get his employees to answer the phone. Of course what I wanted to say and couldn't was "Fire their butts". My point being that you can't get the phone system to do all the work. If he doesn't want to hear continual ringing then he doesn't seem to be worrying about servicing his guests. Even if there is only one desk clerk how difficult is it to answer the darn phone and ask somebody to hold.
If this is a brand name hotel and you go with the auto attendant box on the "Attendant Automatic call forward no answer" you can add a menu item for them to press a digit to transfer to central reservations. Then you can send it to and ext that is Call FWD Always to speed dial which will dial the central reservations number. Just a thought