I had a quit similar issue in a different settings. In our case we
were looking for a solution for both call and conference-call. It was
in a personal context so the financial question was also important
and we needed something stable and easy to use on different OS.
Our solution was Mumble-murmur (http://mumble.sourceforge.net/) which
are BSD and GPL licensed.
The client, Mumble, runs on Windows, Mac OS X and GNU/Linux. This was
an important point for us as we were a quite heterogeneous group.
Furthermore, the Windows client is very easy to install (the windows
users of our group were the most computer illiterate, thus we need
something very easy to use for them).
The server component, Murmur, can run on GNU/Linux even if, for our
part, we use public murmur servers. As Mumble is really common in the
gaming-world there is a lot of solutions to get a channel from
setting up your own server to public hosts (commercial or not). You
have a list of some hosts on the mumble sourceforge page
(http://mumble.sourceforge.net/Hosters).
Have a nice day
