We have multisite which can integrate 5 sites in a IPSec VPN Mesh with extension dialing among sites, but this is not a cluster. It is distributed call control (each site has its own dial plan, AA, voice mail...etc).
Looking a little further up to ISRs may ht the product set you want?
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...