Can you "ping" the machine from outside of the firewall?
Here is an idea. Check the port in the TNS file on the server. You will need to open this port on the firewall to the services you are using, for example TCP/IP.
If those services are not available, you have no connection.
By the way, do you have firewall software like Checkpoint or F5 or is it homegrown?
Good luck.
