Windows 2003 RRAS Pimphony application problem over VPN

Hi, there.
I have a problem regarding a small Alcatel IP telephony program called
PIMphony.
It is working fine on our LAN, but not working over VPN.

We are using WIndows 2003 RRAS as our PPTP-VPN gateway.

The intresting thing is that this application is working fine with an
internet connection of one ISP, but not working with another one's.

I have tested this problem in a test lab with two windows 2003 standard
servers (one is acting as a router -simulating the internet-, other one is
acting as a vpn gateway) and the problem is still the same.

Other TCP/UDP based applications working fine over VPN, only Pimphony has
problem.

I guess it is a router problem, any help regarding investigating this
problem is really appreciated.

Thanks,