Hi Mark,
Go to CUCM enterprise parameters and look for 'URL Directories' under 'phone url paramaters'.
The default url is 'http://<CUCM Hostname>:8080/ccmcip/xmldirectory.jsp'
If the default url has not been altered,you could try changing the 'CUCM hostname' to ip address to eliminate any name resolution issues.
If you want to point to a custom directory menu, you could host your own xml file on a web server and point the url to it.
regards,
Deepu