proxy and vpn

[LiL'Speedy]

Hi there,
I have a problem using a proxy server trough an ipsec-vpn-connection with the iphone.
VPN itself works fine, the iPhone connects to the vpn-server, but the iphone doesn't use the proxy settings given to reach the websites in the networks of our company.
I tried to set proxy manually or via proxy pac, nothing works.
Or did I get this wrong, and I can't configure a web-proxy-access through vpn? (For Wifi it works)

Any ideas? ;-)

Thanks in advance.
Speedy