Today I'll outline how to connect Cisco Call Manager 7 to OCS via Direct SIP. For ease of explanation I am using an environment with 3 digit extensions. The extensions in OCS are 1XX and the Cisco extensions are 2XX.
First, create a SIP Trunk to your OCS Mediation server in CUCM:
Other than selecting items for the required fields, there are only a couple things to ensure. First, make sure to check the box for "Media Termination Point Required." After that all you need to do is set the Destination Address to the FQDN or IP address of your Mediation server and click Save. You will receive a warning about resetting the SIP Trunk - just click ok. Since no one is using the trunk yet it's safe to reset it. Here are some reference screen shots (click on the image to see it full size):
Next, create a new Route Pattern:
In my case OCS is using 3 digit extensions that start with a 1, so I enter a pattern of 1XX. In the Gateway/Route List field I select the SIP Trunk that I just created. Click Save.
You will receive a warning that the Authorization Code will not be activated. Click OK to continue.
That's it for the Cisco side. If you had multiple Mediation servers for different route patterns then you would create a new SIP trunk and route pattern for each. If you have multiple Mediation servers to which you can route the same digit patters then you would use Route Groups and Route Lists instead. Further information on this is available in the Cisco documentation. Now we move on to the OCS side of things.
First, I configure my Mediation server to use Call Manager as the PSTN gateway.
If you have multiple mediation servers you may need to specify the route for the Cisco extensions. In my case I'm using this Mediation server only to connect to Cisco because I have another Mediation server that is already providing PSTN routing, so I specify a route for the 2XX pattern to go through this specific mediation server.
Depending on your AD and OCS topology it may take a little while for the changes to be picked up by all the OCS servers, but once this occurs you should be good to go.