Just a quickie, and apologies for not thoroughly digesting this thread before chiming in....

Your LAN IP network/subnet is 192.168.0.x/255.255.255.0 - VPN will fail (but I can't remember at what stage, and maybe this is client specific anyway) if the LAN you're trying to reach is using the same network/subnet. I'm talking about the office LAN sat behind the remote fixed IP you're trying to connect to.

When connecting via the satellite broadband, the PC interface that's initiating the VPN is 192.168.0.<something>. When you initiate the VPN connection across dialup, the PC interface making the connection is likely to be something completely different (and therefore working).

Posting the results of "ipconfig/all" (from a command prompt window) with and without dialup enabled might give a clue as to whether any of this is worthy of further consideration.

Perhaps more importantly, the results of "ipconfig/all" from a friend working on the remote network would help promote or completely disprove this theory.