Problem with a Billion 7404VGO-M port forwarding

I have a Compaq DS10L happily running behind a Netgear router - port 80
forwarding from the router to the DS10L - all good for about the last 3
years.

Because of VoIP issues, I need to change the router and there isn't a
Netgear available that will do VPN, VoIP and QoS, so I now have a Billion
7404VGO-M. The problem is, on the Billion router, I can forward port 80 to
any server *except* the DS10L - I have pushed it to various Windoze servers
and it works fine, but as soon as I point it to the DS10L - nothing.

Replacing the Billion with the Netgear makes everything work as usual,
except I don't get VoIP operating properly due to QoS issues.

So, anyone have some suggestions? Billion are no help, have already
reflashed/upgraded/swore at the router etc. Key point seems to be, port 80
to anything *except* the DS10L is fine on the Billion, port80 to the DS10L
is fine on the Netgear, so.....