Sure you can... you use call handlers for this typically (although you can also use subscribers).
Each handler has 12 keys you can map (0-9, * and #) such that they take action when the user hits them while the greeting for that call handler is playing. There's a number of actions you can take but the most common is to pass the call to another call handler. In this way you can create "trees" of any depth you want for audio text applications and the like.
Be watching AnswerMonkey in a couple weeks (shortly after the 3.1(2) version releases to CCO) for a graphical audio text manager that will give you a snappy "tree view" for putting such applications together quickly and easily (among other things).
Jeff Lindborg
Unity Product Architect/Answer Monkey
Cisco Systems
lindborg@cisco.com
http://www.AnswerMonkey.net (new page for Unity support tools and scripts)