I have built the sofiasip packages which contain the fix for
https://bugs.maemo.org/show_bug.cgi?id=8615. These packages are especially useful with FreeSwitch pbx server.
Edit:Bumped telepathy-sofiasip from 0.6.2 to 0.6.4. Please note that installing the telepathy-sofiasip would not let you update N900. I can build 0.6.4 package in 0.6.2 packages disguise if anybody is interested. That will let you update the device using HAM or Fapman.