Good thought -- and very worrying! I tried the voice recorder app, though, and it worked swimmingly.

Now I am thinking it might be something to do with Nokia's SIP on Maemo and Gizmo and my local network; I'll give it a shot at a friend's place and see if there's any more luck.

*edit* Okay -- taking Sjgadsby's step by step advice, it'll work when on T-Mobile's data network (at least, it connects, haven't tried a call yet), so it must have been something at my home network. I'll poke around at the ports later and post if I figure it out.

*edit 2* It *was* my home network -- the *modem* my ISP gave me, not the router, had something called "SIP ALG" enabled. From what I can tell googling quickly, this is supposed to make SIP work better... but is often very buggy or just plain wrong, which explains why the more robust desktop based apps might be able to work through it (although Kphone had no audio out) while my N900 wasn't.

I turned it off and now my N900 is working beautifully with Gizmo SIP.