First you should check if your SIP provider really supports the H.263 or H.263+ codec. At least here in germany by far not all SIP providers support codecs for video calls. Most just support audio codecs like G711, G726, G729...
My SIP provider is http://www.dus.net/


The stable version of Ekiga, which you can also find in the Ubuntu repositories, doesn't support H.263(+) video calls yet (only the older H.261 codec is supported). The development version has H.263(+) video support though.
http://wiki.ekiga.org/index.php/Whic...with_Ekiga_%3F
I don't feel the urge to install the SVN version of Ekiga on my Kubuntu machine. So I'll wait until the next stable Ekiga release.

The only other Linux Softphone I could find which claims to support H.263 is Linphone. But Linphone doesn't work for me at all.

On Windows there are more SIP clients with Video support. I have tested X-Lite 3.0 and video calls between X-Lite and the N800 are working just fine.
http://www.counterpath.com/xlite-overview.html

In another forum a user wrote that he is using the Grandstream GXV-3000 for video calls to the N800.
http://www.grandstream.com/gxv3000.html

Forgot to mention: the SIP video option is only available on OS2008, not on OS2007.