Perhaps your modem doesn't use UPnP technology. That means you should manually forward a port (open a port.) I had to do the same thing when I went up from dial up last year.
These steps are very important to follow in order & do carefully:
1. Set up a Static IP address.
http://www.portforward.com/motorola/sbg900-dhcp.htm (& use the link at bottom of that page also)
2. Forward a Port using
Port 6346 See
http://www.portforward.com/motorola/...forwarding.htm
3.
Manual port forward instructions & sample image & 2 posts after that.
Good luck!

(By the way if you were using net games or video conferencing you'd need to do the same thing but open many more ports.)