If we assume that the two companies wish to stick with a Cisco solution then you would need to employ a Cisco VCS Control and VCS Expressway (VCS-E). You may also be able to pull this off with a VCS StarterPack solution (depending on scalability needs and other design goals). Both entities would need to have a solution in place.
At a high level, you would have a SIP trunk from UCM to the VCS. The VCS has a peering relationship with VCS-E and the VCS-E is sitting in your public DMZ. The VCS-E at Cust A can communicate to the VCS-E at Cust B. You could actually do this via a peering model (provisioning on both VCS-E devices) or you can simply leverage DNS and URI dialing to facilitate call setup initiation.
You obviously would have to deal with the network side of things (IP addressing, Firewall rules, etc.). You will need to do some "dial plan" coordination depending on what (if any) URI dialing rules have been established by either organization. With CUCM versions prior to 9.x you should not that URI dialing has somewhat limited support. CUCM 9.0 adds some URI dialing enhancements. Not that you have to be on 9.0 or later to do what you want, just that you have a different set of parameters to work under with the pre-9.0 release.
The above is pretty generic but it may give you a reference point to start with.
HTH.
-Bill (http://ucguerrilla.com)
