By default, Call handlers accept Caller Input. As long as the incoming Caller dials a valid Unity Subscriber's mailbox number, it will route the call to the mailbox. If you want to ring the Subscriber phone, you need to change the Tranfer settings in the Subscriber mailbox to ring the phone. One big rule, the Called part MUST have a Unity mailbox.
In short, place a DID'able number on the Call Handler, record your Greeting and it's done. If you want sub-menu's it can get more complicated but it's really not bad. Just be sure you have a valid path for every option.
I also give a path to have the Caller be able to get back to the Top of the menu and the Top of the Whole Auto-Answer stack so they can repeat the options. Also be sure the '0' out option has a valid path. Everyone '0's out.