It looks like you can't thank posts in the 'Off Topic' forums.

I spent some time trying to get YateClient (the softphone / gui version of yate) working on the n900, and had some success. I got it into extras-devel, but I had issues getting audio to work (and the interface would have to be modified to work on the small screen).

I didn't try, but it should be possible to get the yate server to run on the n900, and then register a sip account to localhost. Interesting idea...