Horacio,
Depending on which version of IPCelerate will determine the answer to 2nd part of your question. The newer version supports unicast (using a remote server) or multicast using the seed server in a central location.
As with any design to better provide an answer we'd need a little more information on the network structure you have. If there are L3 boundaries between your phones in the page group and the IPSessions server and assuming you are not using Unicast mode with a remote server (IPCelerate 6.1 or higher), further complicated by a WAN that doesn't support multicast you have to build out a multicast network. This is commonly done using GRE tunnels with ip pim sparse-dense-mode enabled.
The way the server and CUCM interoperate is via JTAPI. An account is built on CUCM and used on the IPCelerate side to create CTI ports and import the database of phones associated the application user you created in CUCM. IPCelerate actually has really good engineers who are very helpful and their documentation is pretty good as well to overview the installation (in some cases will even help you through it via WebEx).
Call flow is pretty straight forward, user initiating the page will enter the extension of the page zone (you determine the length and pattern and use translation patterns to point to actual CTI or have the users call directly - your choice) when the pattern is dialed CUCM sends the call setup via the CTI port to the IPCelerate server which then initiates a call setup back thru CUCM to the phones in the group and sends a Mute phone and open speaker port. Once confirmation is received that this call setup is completed and confirmation tone (beep by default) is heard on the phone initiating the page and phones in the zone. A multicast (assuming this is the infrastructure) is sent to all phones in the zone. When completed the call tear down process is initiated with the user who initiated the page going on hook.
Now there are some http aspects of the call and some other things going on but for all intents and purposes this is what is going on.
If using Unicast there is no need for GRE if there is a WAN that doesn't support multicast in play.
Hope this helps
