Just to make this thread complete. I got this working by adding a stun server to my internal network (thanks to the maemo guys). After I had added the stun server I went to the advanced settings (for the SIP configuration of the N900) and entered the stun server's address. Now everything is working great.

Obviously you shouldn't need a stun server on an internal network where there is no NAT. Anyhow, it is a work-around that is acceptable to me.