The problem is currently not fixed (even not in community SSU AFAIK), but I was told that there might be a chance to backport fixes from the rumoured upcoming harmattan phone.

In my experience it really depends on the access point, with my fritzbox 7170 as home/office there is no way to make reliable sip calls with the N900, I eventually decided to use my old E51 here (which works perfect for years now). But I had no problems in several Free Wifi Cafés and the conference venue in SFO. Saved quite some money ;-)