Maybe a strange question when usually people ask why things do not work for them... but:

The N900s SIP-implementation is based on the telepathy-framework, right? And it "just works". I never made any adjustment (like port forwarding etc.) to the routers my N900 connects to. I get a call, I hear them, they hear me, all well.

Now on my desktop computer, I use basically the same setup - or so I thought. It's a GNU/Linux system with all the Telepathy-goodness we find on our N900... only the client is different, of course. Instead of Nokias phone client I use Empathy.

Guess what? Don't work. I do receive calls and if I dial out, the other phone rings, but I never hear what the other party says. These are symptoms I know from my early VoIP days and they always had to do with broken NAT traversal.

So I started wondering - why the hell is Nokia's implementation working? Is there somebody who's familiar with how they do things? Is there something they do at the "top" (=within their phone client), outside of the underlying Telepathy components, that eases NAT traversal? How is it possible, technically, if the router isn't prepared for it? What could I be missing on my desktop PC that prevents it from working like my N900 does?

(I checked the packages: Most of the relevant packages I have on my desktop are newer than the ones found on the N900. The only package not available for my desktop is telepathy-stream-engine, which they say is superseded by the other components meanwhile.)