What you will have to do (in your internal DNS) is to create a ZONE named the same as your "SIP" domain in my case visualgov.com since I want to use my users email addresses in OC 2007.
After that you start with creating two A records, one Named "sip" and one named "sipinternal" these two should point to the IP address of your standard OCS server or your enterprise pool.
Second you should create a SRV record looking like this:
Domain: visualgov.com
Service: _sipinternaltls
Protocol: _tcp
Port: 5061
Host offering service: should be the A record (in my case sip.visualgov.com)
After you have created these DNS records you should create a certificate with the Subject name of your FQDN of the server. And a Subject alternative name of the A record (in my case sip.visualgov.com)
This should do it internally.
And the external DNS configuration is just as the guide says it is.
Keith Kabza OCS-MVP
http://www.ocsmvp.com