VPN

I'm using m0n0wall as pptp-VPN server and there are 5 clients which can connect without problems.
Now i want to connect  4 other users who are behind a firewall (big company with a few 100 users).I talked to the admin to open GRE and tcp 1723 outgoing for these users and he did, but when i want to connect i get an error 619.

So i phoned the admin again and he told me that he can watch packets going through his FW (1723 and GRE outgoing). He added that the Problem must be, that our m0n0wall answers on random ports. His firewall only allows 1723 and GRE as answer.
I also tested this with wireshark and the monowall really replys on random ports (between 1000-50000).

Is there a way to config the m0n0wall so that it answers on port 1723 and GRE instead of answering on a random port ?

sry for my englisch and thanks in advance.

greetings