Clients can’t connect to VPN server over

VPN server is running Win2k. It is connected to internet with static ip. All ports are opened on firewall for that address. Client can’t connect to VPN server via ISDN line. It is an Lucent router running NAT. Clients that use dial up connections to the internet and the launch VPN connection work fine. I think the problem might be NAT running on the ISDN router. Can this work?