I downloaded the X-Ten softphone and was suprised to find out that the video call did not work.  When trying to enable the video it would turn on the "Sending video" message and then turn off again.  Searching through the docs and some google searches revealed the problem.  Video support is not turned on by default in the asterisk setup on trixbox. 

Here is the steps to enable video aupport. (I am running trixbox CE 2.6 and asterisk 1.4):

Edit the following file /etc/asterisk/sip_general_custom.conf and add the following lines

videosupport=yes
allow=h264
allow=h263
allow-h263p

Then restart asterisk so that it reloads the config files.  You should now have working video support.

NOTE: X-Ten seems to have a bug in their code (or asterisk I'm not sure). When you start sending video, only the person that received the call will see both cemeras.  The fix is to put the call on hold for a second and then back off hold, you should now be able to see the video on both sides.