Not sure why then your ISP is claiming you need a static IP for it to work, because I use iChat and do not have a static IP.
What speed of connection are you on? Ask your ISP to define/clarify why you would need a static IP, when many people who have dynamic IPs can use it without problems.
It does sound like port blocking for dynamic IPs to be honest, but as you have sorted that one, I would ask them their exact reasons why you need one.
If it works with another ISP or on campus, then the fault lies with your ISP who seem to be restricting bandwidth for some reason, unless your connection is slow or you are on dialup.
EDIT: If you are using a router and have NAT enabled it could be causing the problem, (owever, do not disable NAT!) Below is a link giving information about possible problems/compatible products and also it lists all the ports that should be open.. If you are running a firewall, then you need to add these ports to the firewall rules if the program is not listed:
http://docs.info.apple.com/article.html?artnum=93208