I'll assume that you want to create some sort of custom directory. The phones can browse the CallManager directory already (standard CallManager functionality), so are you wanting to customize this somehow, or attach to a different directory? Here are some thoughts to get started with... may or may not be relevant depending on what you want to do..
First, get the IP Phone SDK from Cisco Developer Support:
http://www.cisco.com/pcgi-bin/dev_support/access_level/product_support
The programming reference guides are also useful:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_programming_reference_guides_list.html
The CallManager hosts an LDAP directory with users and their devices, extensions, etc. So to develop this you'll want a CallManager and an xml-capable IP phone (such as the 7905, 7912, 7940, 7960, 7970).
Depending on what language you will be implementing you service on: Check out the ldap application in the ASP folder from the IP Phone SDK, or something like the photodirectory app from the JSP folder of the IP Phone SDK.
Note that your service will be hosted on a web server, not the CallManager (Cisco does not want any 3rd party applications running on the CallManager). Your web server will communicate with CallManager using LDAP and possibly HTTP, and with the phones using HTTP. (the phone requests the directory from your web server, and your web server requests directory information from CallManagers LDAP directory).
Not sure if that's what you're looking for...
One last thing: since you mentioned IP addresses: If you want to know the IP address, devicename, or model number of the phones capable displaying XML, then check out the devicelistx report. Samples are available in the IP Phone SDK, or in the programming reference guides from the links above... or this shortcut:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_programming_reference_guide_chapter09186a0080405760.html
