redhat web server not connectable

Hi,
I am running tomcat5 on a linux Redhat9 machine on port 8001. And I am
using cable service to get on line.
Tried to access the web server from the outside network but the result
was inconsistent. I do have the iptables running.
Now I get to the point that I can access the web server through some
"anonymous surf" service like "Proxify" or "Guardster" but I can't
access by directly using xxx.xxx.xxx.xxx:8001.

netstat -an shows:
Proto Recv-Q Send-Q Local Address Foreign Address
State
tcp 0 0 0.0.0.0:8001 0.0.0.0:*
LISTEN

tcpdump captures the traffic when using the anonymous surfing proxy
but nothing when trying the direct access to port 8001. I tried the
different port and the result seems to be consistent.

I am suspecting the ISP blocks the http request but not sure if they
can do this on all the ports besides the populer ones like 80, 8080
.... etc.. And why are those proxy services able to get through?

Can some shed some light on this?
Thanks.
Ren