It's not really possible to look up against the CCM end user directory, as it's not even used for internal extensions - the calling name is populated from the DN configuration (alerting/display).
CUPC can do reverse lookups against LDAP and runs on your PC, but requires a Presence cluster to go with your CCM
CUCiMOC can do similar things if you have OCS deployed in your organisation
Otherwise if you really need it on the phone try http://www.ciscoscripts.com; another contributor on these forums has some TCL scripts you can run on your gateways to get this functionality.
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