I wrote this several years ago for a book, but it still applies to recent versions of Unity as well - on the Documents page of www.CiscoUnityTools.com you'll find a paper called "Audiotext Applications in Unity" - it was a chapter in the "Unity Deployment and Solutions Guide" from a while back:
http://www.ciscounitytools.com/Documents.htm
give it a look and see if that gets you started.
You can drop WAV files into greetings for call handlers using the media master control in the web SA - the little playback widget there has a drop down to import from a wav file on your HD.
You may also want to eyeball the Audio Text Manager app which can make stringing these types of handler trees together a bit easier:
http://www.ciscounitytools.com/APP_AudioTextManager403.htm
There's a short training video on its home page that might help clear a few things up as well.
If you run into problems after reviewing that material, let us know and we'll see what we can do.