It doesn't happen when calling to other cisco endpoint.
I tried to call cipc, jabber on windows, telepresence server and all works well either video or audio.
I also tried to call from another 9971 phone, and the same problem occurs.
There is also a polycom mcu connected to VCS.
Video call to MCU works well.
I also tried to register other sip clients like x-lite and ekiga to VCS,
But call from 9971 to those endpoints is automatically switched to phone call, not video.
yet, the video capability is already enabled.