PPTP over Double NAT

I have a linux box that is running PoPToP and I can log in fine. It is
sitting behind a FreeBSD box with a realworld IP address. This BSD box
is supplied my ISP and I have limited access to it but it's set up to
do the NAT and allow everyone behind it internet access.

I am able to login to PoPToP from within the local network or from my
DSL connection at home fine. My DSL connection has a Linksys router
with NAT to my local network.

The problem is when I try to connect from another BSD box the same as
above. I have been talking to the ISP and they have told me that it's
because PoPToP dosen't send the correct Call ID headers to allow the
double NAT to work. I don't fully understand what they have told me..
but it sort of makes sence.

They have also said that they have other clients that use the same
setup with a M$ PPTP server with no problems..

Any Suggestions?