If it helps anyone I test my call quality using these automated test numbers:
Info here: http://thetestcall.blogspot.com/. They test latency, DTMF, recorded sound quality, etc.
For what it's worth I get better quality over the first number for some reason. Maybe it's closer to me. So, actually, I use the "bad" one as a good indicator of whether my setup will be acceptable in various circumstances.
An IPKall number has recently been added (which should also perform better than the 703 number):
Feedback appreciated! (Option 0 in the menu)
Thanks to M Seuss and gggirlgeek: this is very helpful info!