For CUCM to CUCM trunking, in most cases, you would use an InterCluster Trunk. This uses Cisco protocols to handling the trunk connection between the two systems. A SIP trunk is simultaneously more complex and too simple; It is a protocol designed to allow any device that adheres to the SIP standard to speak to any other devices that adheres to the SIP standard. Problem is, there's a lot of opinion about what exactly the "SIP standard" is. For Cisco-to-Cisco connections, I would use the Cisco propritary solution.
Now, if you are going to be connecting a number of CUCM's together, that raises other issues. You run into the "handshake problem". You need to build and configure a connection between every two CUCM clusters. This leads to N*(N-1)/2 connections, which is quite managable when N is 3 or 4, but becomes a hassle when N=10, and impossible when N=100. That is when the Gatekeeper comes into play. The Gatekeeper allows you to setup each cluster once, in the Gatekeeper, and the Gatekeeper plays traffic cop; Each cluster sends any non-local, on-net traffic to the Gatekeeper and the Gatekeeper directs it to the correct cluster. It handles address resolution and call admission control, as well.
But to get started, all you need is to setup an ICT is the IP address of both ends.
