Hotline still exists?
Ah, those were the days, running my own Carracho server (gronk-o-matik™)...
(Hotline sucked major ass, even then)
Anyway, the ports that need to be open in any Firewall are these (googled for "Hotline port numbers"):
Hotline Server
IN TCP 5500 - 5503
IN UDP 5499
The TCP Ports enabled are 5500 - 5503 (This is for the standard 5500 Hotline port)
If you change the default port, then you must enable the 3 ports after it (so if you choose 4000 then you must enable 4000 - 4003)
The UDP port enabled 5499 is required only if you want to list your server on a tracker (the data stream is only outgoing so if you want to disable in bound on a firewall it would work fine)