Maximum PC

So I am experimenting with a PBX server virtualized on my machine. I have it up and running following this guide. The server is connected to the internet and is not behind a router. The problem is that I can't seem to connect to the server with any kind of softphone. When I try to do it through the internet, I get a timeout error, but when I do it over Hamachi, I get an "SIP error 503". My school hands out public IP addresses for each device that connects on its network, so im 99% sure its not a NAT or port forwarding problem. I would really appreciate the help as this is just incredibly frustrating.