The latest version of the LANdialler client and server is currently only available from CVS. It uses GTK2 and has (so far) only been tested on Linux. It works very well, but there are a couple of minor bugs to be flattened before version 0.3 is packaged. If you'd like to try it check it out and run the client and server from within the checked out directory.
Older (and fully tested) versions of LANdialler are still available from the project files page. Note that they have been written to work with Python 2.1, GTK1 and Tk, and have not been successfully tested with Python 2.3. Installation instructions are included with the source code.
The client should run on any operating system with Python and GTK/PyGTK support - it has been tested on both Windows and Linux.
The server should run on any POSIX compliant operating system, but has only been tested on Linux. You could use any of the BSD variants (e.g. FreeBSD, OpenBSD, NetBSD) or Solaris, etc. if you prefer (success reports would be very welcome). Just make sure you can configure it to route IP packets between your network and the Internet.
There is lots of documentation available for Linux if you're not sure how to set it up.