Login to a Cisco GK through VPN 3005

We are doing some tests with a Cisco VPN 3005 concentrator that is connected with the folowing scenario:
Public interface directly to internet and private interface to DMZ of a Checkpoint Firewall-1.
We had some troubles to establish phone call (ip to ip) with netmeeting with vpn client v. 3.6, now, with the 4.0 version that fix a problem with netmeeting we are able to call from or to internal network with the vpn client who tipically is connected to internet using dial-up or broad-band.
The point now is that we can not login in with our internal gatekeeper (cisco router) from the vpn client using netmeeting.
Firewall is properly configured, also, if i replace the private interface of VPN 3005 with a notebook i am able to login in to gatekeeper from DMZ to the GK placed in the internal network, so i assume that the problem is in the box.
The client is getting an ip address from the DMZ segment wich is NOT NATed in no box from the concentrator to the Cisco GK.
When one of this ip address is used in a notebook connected to the dmz segment it is possible login to GK.
If some one had been experimented with similar configuration i will thank a lot some feedback.
IP voice call from ip to ip is working fine (initiated by the vpn client or from an internal network user to the ip address of the vpn client), but i would like use the GK to access to our voice network.
More extrange is that if i use openphone from the remote user’s notebook that is connected with the cisco vpn client, openphone v.1.8.1 is able to login to gk but i can not make calls, the same happen from the internal network using openphone.
So, it is a little difficult identify the cause of problem, in the first situation, the vpn 3000 appear like the problem, but due that openphone can register with the gatekeeper i can not find a clear answer to this scenario.
Thanks in advance, Martin.-