Hi Sandeep,
There are a few things to check here. First, verify that you have performed a Manual Synchronization of Directory Services in UCCX. This is usually the one that takes care of the problem. Go to Cisco Desktop Administrator and under "Services Configuration" see the entry "Synchronize Directory Services". There is a button there you click to manually Synchronize. If the first attempt fails, try again until you see a "Transaction Complete" message.
If that didn't work, ensure the extension number has been added to the list of controlled devices under rmcm application user in CUCM. If the IP 7942 device wasn't previously added, add it and save your changes.
If that didn't work, ensure the extension number has been added to the agent's end user profile - more specifically, make sure the MAC address of the IP 7942 is shown under the profile for "Controlled Devices" and also make sure the correct extension number is listed for the IPCC extension under "Directory Number Associations" in his profile.
Usually I add agents in reverse of the steps listed above by first adding phone MAC and IPCC Extension to end user profile, then adding MAC/DN of phone to rmcm application user and then finally performing a manual synchronization of directory services in UCCX.
Hope this helps,
Scott