Connectivity with OCS 2007 from outside the network is accomplished with the help of an OCS 2007 Edge server which is essentially the DMZ gateway that sits between the internal and external network to allow secure communications from outside. The Edge server can be configured with a certificate and set up to communicate using SSL (Secure Sockets Layer). You can learn a great deal more about the security of the OCS 2007 Edge server from this Microsoft white paper.
My follow up answer though is "it depends". If you are not using Microsoft Unified Communications, much of my explanation above may be meaningless to you. However, even with other vendors there is often some sort of similar approach.
This was first published in October 2008