I looked through the archives and this problem hasn't been reported since March of 2011. I can do SIP registration just fine, and even receive calls when the app is in the foreground, but when the app is backgrounded I don't get anything.
Also the app outputs no log information when the app is backgrounded, so I'm unsure how to go about debugging this. Any thoughts on what to look at? The network settings are all the defaults, UDP, default port, etc.