The feature you are asking for is now available in Unity 4.1.1 and can be enabled on a unity account basis to allow a subscriber calling in from the unity extension # or a configured alternate extension to not have to enter the phone password. All ...
Under Service Parameters | Callmanager set your National Number Prefix to prepend the digits you need. Normally this is a 91. All incoming missed calls will then be able to Dial without editing.
