I've tried your alpha-3 release on my Intel Mac (OS X 10.5.1, Java 1.5.0_13), and I had a few problems with it. Here are my comments:
Download DMG, start application to try it out: crashes without error message or anything (are you trying to write into the .app bundle rather than writing to ~/Library/Preferences/...?)
Account Registration Wizard: trying to register, Apple-V doesn't work.
Trying to make a call to a +44 number via my account on sipphone.com.
Connection failed, but not before I've seen some strange sounds on top of a normal "calling" tone. It also ays "Incoming Call", which is confusing!
After saying "ok" in the error message, my speaker keeps outputting unpleasant noises.
Let's try to dial again - but the call history is empty. I would expect to find my number in there!
Trying to configure the program, but the only menu I can find is "SIP Communicator", and it's empty. Apps should have menu bars on the Mac... It only takes a parameter setting for a Java program to move the menus from the window to the menu bar.
In the Account Registration Wizard (why a wizard? I'm not used to wizards on the Mac, and we're in the Preferences dialog, where I wouldn't expect a wizard!), under Avanced Options, there's a Registrar field, but I can't enter anything, even if "override server defaults" is selected (it's unclear what that means, since I haven't told it what my SIP server could be - which server does it refer to?)
Even with the proxy set, I can't get the connection to work. In the error message, it says "... Server:.", suggesting that there is no server set. But where do I set it?
When I went back to the Preferences (the wizard), the "override server default options" had magically unchecked itself.
OK, enough playing around - I hope my notes help you improve the program!