(x-posted from "USB Device Port" forum)
We use a Vodafone E172 3G dongle as broadband backup on our Vigor 2800 in head office. The failover works well for email & web surfing - after 20 seconds we are back online over 3G.
However we cannot get our PSK IPsec LAN to LAN VPN working with the 3G.

The 3G site is not getting a proper public IP address from Vodafone:
The router displays the IP as 10.x.x.x ... but when I use whatsmyip.org it says something different : 212.x.x.x
Currently the Dial-In end is 3G; The Dial-out ends are ASDL
But it's LAN to LAN, so we could easily set it up the other way round.

Anyone know what we should be doing?

> Currently the Dial-In end is 3G; The Dial-out ends are ASDL

That's probably the problem then; Vodafone are operating NAT and presumably not sending unsolicited packets received at the public IP address onto your private one - that's like you not setting up a port redirection or DMZ on your Vigor...

I think you just need to reverse it so that the 3G end dials out to the ADSL site.

The 3 network would provide you with a routable IP address.

I tried reversing it (as suggested above) but no joy.
The packets are obviously not getting back across the Voda network.

I have now given up on this.