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.
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