I happily use SIP on Diablo with the built in client. For it to work, though, I had to forward UDP ports above 1024 to the N800 - all UDP ports. I know from other SIP-clients that usually the client sets a number of ports it will use for RTP, and it would be enough to forward exactly this port range. So does anybody know which ports SIP on OS2008 uses for RTP and maybe even how to change this setting?