Free World Dialup looks interesting. A few clients are availible. FWD eyeP Phone Lite from eyeP Media was the only one that worked for me. One of the other two didn't bother getting me a # (it kept saying it was trying) and the other wouldn't connect to the system.
My sound is very choppy, but I can hear the other person quite well. Also, I need to figure out how to stop the feedback from the speakers.