Excerpt from following link: 

"Firewall systems help prevent unauthorized access to computer resources. If a firewall is turned on but not correctly configured, attempts to connect to SQL Server might be blocked.

To access an instance of the SQL Server through a firewall, you must configure the firewall on the computer that is running SQL Server to allow access. The firewall is a component of Microsoft Windows. You can also install a firewall from another company. This topic discusses how to configure the Windows firewall, but the basic principles apply to other firewall programs."

There a more ports than TCP 1433 that need to be opened - read complete document at:

http://msdn.microsoft.com/en-us/library/cc646023.aspx