Im using the nokia native sip client on n85. Work very good to make calls, much better than any other Sip client for symbian.

BUT:
Not reconnect easily, so no one can callme through SIP to my account. So, this plugin in practice, only work to make calls and not to receive.

I always run on 3G only. But when loose connection, Nimbuzz or Fring try again to connect. Native Sip Client always fail to reconnect.

Yesterday I downloaded the QT SDK, Im tired of waiting someone to make a good XMPP client with jingle-voip support for symbian. Wishme luck!