The best place for you to start is the administration guide for Unity 2.x - you'll find all the administration guides for all versions of Unity on the web here:
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_administration_guides_list.html
Short story is you'll need to go into the SA, go to the opening greeting call handler and record the greeting you want (standard, off hours, alternate...) using the media master control on the web page which lets you use the phone or a local microphone for this. The admin guide will get you the detail you need here.
