Good sound starts with good, well-placed mics. Can you confirm that this is what was done? Your speakers have to use either quality boom mic headsets or clip-on lag mics plugged into the computer's sound card. If they are using a telephone conference call mic on an office phone, placed more than a foot from their mouth, you're going to get garbage, and there's not a lot that can fix it, IMO.
As far as telephone quality audio, the guys in the Sound forum will be of more help, but part of the problem is that POTS type phone connections are lossy and cut out a lot of the high and low end to reduce bandwidth and improve clarity of the human voice part of the spectrum in the middle. It will help a *little* to add back some high and low end with an equalizer plug-in, but only so much, because you can't enhance audio frequencies that have been removed during the recording. If you spend a LOT of time noodling around with multiple copies of the audio track, artificially creating a high end and low end, maybe it could sound better, but that's a heck of a lot of work.