iCal and Proxy

In my network we have to use a proxy-server to access the Internet. I subscribe to a calendar published on a .Mac account. When trying to Refresh I get a timeout for webcal://homepage.mac.com/... My network administrator tells me he does not see iCal querying the proxy-server. So, how do I make iCal use the proxy-server instead of trying to directly access the Internet? Browser and eMail clients work fine, as the Network settings are set to use the proxy server.

Thanks in advance for any support.
Uwe