Take a look at the IP Phone programming guide for the syntax of the various XML elements that you can display to the user:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_programming_reference_guide_chapter09186a0080406727.html#wp1033366
Phone applications are often implemented as a series of page requests from the user, with the phone returning conent to the phone based on the URL and parameters of the request. So, for example, the user requests your service, and you display an input form (a CiscoIPPhoneInput structure) in XML. The user can use the phone's keypad to enter the values for the input items, and when they submit the page you can display a new page based on their inputs, etc.
The CiscoIPPhoneInput element allows you to request information from the user. It uses Get, not Post method. The user fills in the values you request, and then submits the page to your web server with their input values attached to the URL as query parameters.
The ASP is fairly straightforward.. set the contenttype to text/xml, and output the XML the same way you would output an HTML page. There are a number of examples in both ASP and JSP/Java that have been posted previously in these forums (IP Voice and Video Developer), try searching the forum using the search function on the top right.
Looking through the example applications in the IP Phone SDK is probably one of your best options.
http://www.cisco.com/pcgi-bin/dev_support/access_level/product_support
In addition, you could check out the book Developing Cisco IP Phone Services: A Cisco AVVID Solution, published by Cisco Press. It's old, but is still a decent "getting started" guide.
That book is available on O'Reilly Safari (books that you can read online for a monthly fee), and Safari has a free 14-day trial.
http://safari.oreilly.com/?x=1&mode=section&sortKey=title&sortOrder=asc&view=&xmlid=1-58705-060-9/d1e2496&g=&catid=&s=1&b=1&f=1&t=1&c=1&u=1&r=&o=1&n=1&d=1&p=1&a=0&page=0
