Thanks for your tips. Yes, I hosted a dedicated server which appeared in the Gamspy listing. So, there should no firewall or proxy problem. I now believe it may be the wrong port we are using (2234). We will try the default port 2302 instead and see if it works.
Czesanne
BTW: Cool project, your CoC. I'm a senior software developer and specialise in "real" AI (automated knowledge acquisition). The OFP AI is really amazing.