Audio Conferencing Using Linux?

Jon Scobie asks: "I work for a small consultancy company with no fixed abode. Our guys are generally based at a clients premises or at home. We try to use technology to make the business work but one stumbling block I've come up against is the ability to do audio conferencing without buying expensive hardware solutions. Basically, we have a Linux server on the net which provides us with web facilites, email, groupware etc. but I can not find a solution on Linux for providing software audio conferencing. It doesn't have to be free, just a resonable price. There is only one solution which I can find (from a company called CUSeeMe) but the price is outragious. Is there something out there which I can't find or is there a gap in the market here for someone to exploit?" If just basic communication is what is necessary, why not run an IRC server, instead of hacking together a (potentially) expensive audio conferencing solution? You can use any internet connection and IRC software for a wide variety of platforms is available, and you can communicate to your co-workers from wherever you can dial-in from. With that suggestion aside, what audio conferencing software is available for your flavor of Unix?