Yeah, you seem to be right. Although I can now call the Windows client from the N900, I still can't get it to call from Windows. I'll try port forwarding tomorrow.

Also the reason for the question is not up-to-date anymore I would like to keep this thread open to add additional possibilites + news.
I will edit the first post and add a list of the possibilites posted in here so everyone can have an easy and fast overview of available phone/video/(chat) programs - so of you find any new apps/programs - please share your findings.

I just read about oovoo - altough this also seems unencypted i will add it to the list.