vpn conection to raspberry over pppd

[robert rottermann]

Hi there,

I would like to build a device out of a raspberry and a fona gprs modem.
This I want to set up in a remote place.

now I would like to set up a vpn connection from my ubuntu box to that 
remote raspberry.
Te remote device gets an internet connection from the mobile carrier 
(swisscom).
Unfortunately the internet access it gets is trough a proxy an it only 
gets a private ip address in the 10.10... range.

Now I would like to set up a vpn between my ubuntu box and the remote 
raspberry of which I do not know the io addres.
I do have a publicly accessible server, so I could have the remote 
raspberry store its ip address on it.

However this still does not help as this is an address behind the proxy 
and not accessible from the outside.

any ideas how to start tackling this challenge is very much appreciated.

robert