SBS2003 VPN Access

I have a Windows SBS 2003 network configuration behind a Netgear firewall. I have setup the port forwarding (port 1723) to the small business server.

The VPN connection works FINE as long as i connect to it from a live IP address based connection (ie im not behind a firewall) So my Wireless Mobile connection works fine.

What I want to know how to configure, is for this to work if I am behind a firewall, at a clients for instance. It’s also possible that the network / nat i am on at clients could be running the same IP range.

I know there has to be a way to do this, I just am yet to find it.