Every port IS open by default - i.e. the ports are not actively blocked whether there is anything listening on the port is another matter.
Regards the port 1433 are you sure it's closed ? - have you tried telnetting to it ?
If it IS closed and there's nothing on your or your providers network blocking it have a look in SQL server properties for which network protocols have been installed - it might be that tcp/ip was not included (though it is by default) or that it's set to listen on a different port.
If this server is open to the world you should think about moving it to a non-standard port anyway before the next slammer appears.
Ah, one more thing - how are you connecting to the server (?) - some ISP's also block 1433 so if it's a customer or something that can't connect that could be the reason.
1433 seems to be like a swear word to most network admins