Netmeeting problems

When I tired to connect an internal PC(link to network) to an external PC(dial-up modem type), the connection always failed with this message’ The person you called is not anle to accept Netmeeting call’ appear on the external PC.

But if I use aexternal PC(dial-up modem type) and connect with internal PC(network type), the connection was successful.

I have already open up all the port on the proxy server as stated at microsoft website.

Need solution for this problem.Anyone??