So it doesn't seem to be what I had thought. Here's a couple of things to try (if you haven't already checked those):
-Have a look at http://www.voipuser.org/forum_topic_6100.html - maybe these parameters will help:
sip nat ip: [your router's WAN address]
sip nat port: [the SIP port you forward to your phone - should be 5060]
sip rtp port: [the RTP port forwarded to your phone - 3000, as I gather from your posting]
-Check if your 480i registers with Sipgate. Log into your Sipgate account and check if it considers you "online". If it does, follow the "details" link to see at what IP address and with what type of equipment it sees you.
My advice is based on my experience with other hardware. Maybe somebody with experience with the same hardware could shed some more light on the issue. Short of that, I would probably have a closer look with wireshark or tcpdump. Sorry, no other concrete theories right now...