An update:
I got Kphone working by adding the FWD domain (fwd.pulver.com) in the proxy, setting STUN to TCP, and changing the audio prefs to dev/audio. This allowed me to make calls to 613, the echo test, and 8502, the voice mail. The quality was pretty good until I started testing the range of my AirPort.
The bad news is that I wasn't able to receive calls from the test number, 513. The call -- well, three calls, actually -- appeared as "missed" in my account, even though I was ready and waiting for it. Has anyone had this same issue and figured out how to resolve it?