What IOS are you using? Are you using software pack 7.1.X? If so, can you try to point your CUE module to an external NTP server instead of the UC500? We have a known issue with NTP synchronization and you could be facing it.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...