Had an interesting engagement trying to get a Voice Elements IVR platform to use a Cisco CUCM 7 server as the connection to the PSTN.
The plan was to accept incoming calls via PRI(s) registered to the Cisco CUCM server, and pass the caller to the Voice Element server to be answered by home grown IVRs. The Voice Elements server will also be hosting applications that generate outbound campaigns, signalling CUCM to pass the call to the PSTN.
The Voice Elements set up is fairly basic, requiring defining the IP address and port on which the Voice Elements server software will listen for incoming SIP traffic, and the IP address and port that Voice Elements will use to send traffic to.
You can find the details here: http://support.voiceelements.com/index.php?title=How_do_I_connect_to_my_SIP_Carrier.
The only significant change to the CUCM server was a requirement to use UDP as the transport. At least on CUCM version 7, it appears that the default is TCP.
You can modify that via System | Security Profile | SIP Trunk Security Profile.
I suggest making a copy of the default profile, modifying the Outgoing Transport Type, and applying it to the trunk.
Here's a screenshot of my working config.
If you need any additional details, let me know.